Add a language option to the VR menu, and fix a crash switching to it

- New "Language" row on the Main Menu tab (English/French/German),
  backed by a new "general" group in questshock.config.json
- Fixes a device-reproduced crash: switching language used to call
  language_dealfunc(), whose render_run()/opanel_redraw() repaint
  assumes the game's own in-game options panel is on screen - crashes
  (SIGSEGV) when triggered from our independent VR overlay instead.
  Now calls language_change() directly, which is all that's actually
  needed to reload resources and refresh the inventory/MFD/side-icon UI
- Fixes a second crash: this data set ships frnstrng.res/gerstrng.res
  but not mfdfrn.res/mfdger.res, and language_change() closes the
  current resource files before discovering the new ones don't exist -
  leaving no valid resource file open and crashing on the next lookup.
  Now checks both files exist before ever switching, refusing the
  change (with an in-headset popup) instead of corrupting resource state
- extract_assets.sh can now also pull mfdfrn.res/mfdger.res out of a
  System Shock: Classic installer (which includes them, unlike Enhanced
  Edition) if one is present in res/assets/ - entirely optional
- Documented all of the above in README.md/CONFIG.md

android/app/src/main/cpp/qs_config.c

@@ -284,9 +284,15 @@ typedef struct {
     char resolution[16]; // "" = not present in the file
 } QsParsedVideoOptions;
 
+typedef struct {
+    char language[16]; // "" = not present in the file
+} QsParsedGeneral;
+
 typedef struct {
     QsParsedVideoOptions video_options;
     bool has_video_options;
+    QsParsedGeneral general;
+    bool has_general;
 } QsParsedConfig;
 
 static void qs_parse_video_options_field(const char *key, QsJsonParser *p, void *ctx) {
@@ -299,10 +305,20 @@ static void qs_parse_video_options_field(const char *key, QsJsonParser *p, void
         qs_json_skip_value(p);
 }
 
+static void qs_parse_general_field(const char *key, QsJsonParser *p, void *ctx) {
+    QsParsedGeneral *out = (QsParsedGeneral *)ctx;
+    if (strcmp(key, "language") == 0)
+        qs_json_take_string_field(p, out->language, sizeof(out->language));
+    else
+        qs_json_skip_value(p);
+}
+
 static void qs_parse_top_level_field(const char *key, QsJsonParser *p, void *ctx) {
     QsParsedConfig *out = (QsParsedConfig *)ctx;
     if (strcmp(key, "video_options") == 0 && !p->error && p->p < p->end && *p->p == '{')
         out->has_video_options = qs_json_parse_object(p, qs_parse_video_options_field, &out->video_options);
+    else if (strcmp(key, "general") == 0 && !p->error && p->p < p->end && *p->p == '{')
+        out->has_general = qs_json_parse_object(p, qs_parse_general_field, &out->general);
     else
         qs_json_skip_value(p);
 }
@@ -442,6 +458,117 @@ static const char *qs_current_resolution(void) {
     return "640x480"; // shouldn't happen - safe fallback for Save
 }
 
+// ===========================================================================
+// General: language
+// ===========================================================================
+
+// Live-appliable: language_change() (below) is the resource-reload half of
+// what the game's own options menu calls when the player changes language
+// there (see wrapper.c's language_dealfunc(), wired up via
+// multi_init(..., REF_STR_Language, ...)) - closes/reopens the
+// language-specific resource files, updates which_lang/QUESTVAR, and
+// refreshes the inventory/MFD/side-icon panels' own language-dependent
+// text/art. Deliberately NOT calling language_dealfunc() itself: its extra
+// render_run()/opanel_redraw() repaint step assumes the game's own
+// in-game options-panel widget is the thing currently on screen, which
+// isn't true when this is triggered from our independent VR overlay menu -
+// confirmed by a device crash (SIGSEGV, null pointer) inside render_run()'s
+// 3D-view frustum-clipping code when called this way. The game's normal
+// per-frame render loop already redraws everything anyway.
+#define QS_LANGUAGE_REQUIRES_RESTART false
+
+// wrapper.h declares this one, but including the whole header pulls in
+// other prototypes referencing types this file has no other reason to need
+// - same reasoning as screen_draw() above.
+extern void language_change(uchar lang);
+
+// gamestrn.c/init.c - declared locally rather than pulling in their
+// headers, same reasoning as language_change() above. language_change()
+// itself indexes these two by lang (0/1/2) to reopen the string/MFD-art
+// resource files for the new language - but it does so destructively,
+// closing the *current* (working) files first, with no fallback if the
+// reopen then fails. qs_language_resources_available() below checks both
+// files exist *before* ever calling language_change(), so a language whose
+// resource pack wasn't fully installed can't leave the game with no valid
+// resource file open at all.
+extern char *language_files[];
+extern char *mfdart_files[];
+
+static const struct {
+    const char *name;
+    short language;
+} kLanguages[] = {
+    {"english", 0},
+    {"french", 1},
+    {"german", 2},
+};
+#define QS_NUM_LANGUAGES (sizeof(kLanguages) / sizeof(kLanguages[0]))
+
+// Confirmed necessary on a real device: this project's own Android data set
+// ships frnstrng.res/gerstrng.res (language_files[]) but not
+// mfdfrn.res/mfdger.res (mfdart_files[]) - selecting French or German used
+// to call language_change(), which closes the working English resource
+// files before discovering the new ones don't exist, leaving no valid
+// resource file open at all and crashing (SIGBUS, null resource pointer)
+// the next time any resource lookup happened.
+static bool qs_language_resources_available(short lang) {
+    FILE *f = fopen(language_files[lang], "rb");
+    if (!f)
+        return false;
+    fclose(f);
+    f = fopen(mfdart_files[lang], "rb");
+    if (!f)
+        return false;
+    fclose(f);
+    return true;
+}
+
+// Defined further down, alongside qs_show_restart_dialog() (same
+// find-class-via-classloader JNI plumbing) - forward-declared here since
+// qs_apply_language() above is defined earlier in the file.
+static void qs_show_message_dialog(const char *message);
+
+static bool qs_apply_language(const char *value, bool live) {
+    for (size_t i = 0; i < QS_NUM_LANGUAGES; i++) {
+        if (strcmp(value, kLanguages[i].name) != 0)
+            continue;
+        if (!qs_language_resources_available(kLanguages[i].language)) {
+            WARN("%s: general.language=\"%s\" - resource files for this language aren't installed "
+                 "- not switching",
+                 QS_CONFIG_FILENAME, value);
+            // Only when there's a menu to show it in - live=false is the
+            // startup path (qs_config_load_startup()), before the menu
+            // overlay exists; qs_show_message_dialog() would just silently
+            // no-op there anyway (MenuOverlay.instance is still null), but
+            // there's no point trying.
+            if (live) {
+                char message[256];
+                snprintf(message, sizeof(message),
+                         "Can't switch to %s: its resource files (%s, %s) aren't installed in "
+                         "res/data/ - see CONFIG.md.",
+                         value, language_files[kLanguages[i].language],
+                         mfdart_files[kLanguages[i].language]);
+                qs_show_message_dialog(message);
+            }
+            return false;
+        }
+        bool changed = gShockPrefs.goLanguage != kLanguages[i].language;
+        gShockPrefs.goLanguage = kLanguages[i].language;
+        if (changed && live)
+            language_change((uchar)kLanguages[i].language);
+        return changed;
+    }
+    WARN("%s: general.language=\"%s\" is invalid - ignoring", QS_CONFIG_FILENAME, value);
+    return false;
+}
+
+static const char *qs_current_language_name(void) {
+    for (size_t i = 0; i < QS_NUM_LANGUAGES; i++)
+        if (kLanguages[i].language == gShockPrefs.goLanguage)
+            return kLanguages[i].name;
+    return "english"; // shouldn't happen - safe fallback for Save
+}
+
 // ===========================================================================
 // Read-only queries for the Video Options tab (see qs_config.h)
 // ===========================================================================
@@ -483,17 +610,24 @@ static void qs_config_apply_parsed(const QsParsedConfig *parsed, bool live, char
                                     size_t restart_needed_size) {
     if (restart_needed && restart_needed_size > 0)
         restart_needed[0] = '\0';
-    if (!parsed->has_video_options)
-        return;
-    if (parsed->video_options.renderer[0] != '\0') {
-        bool changed = qs_apply_renderer(parsed->video_options.renderer, live);
-        if (changed && QS_RENDERER_REQUIRES_RESTART)
-            qs_append_restart_reason(restart_needed, restart_needed_size, "renderer");
+    if (parsed->has_video_options) {
+        if (parsed->video_options.renderer[0] != '\0') {
+            bool changed = qs_apply_renderer(parsed->video_options.renderer, live);
+            if (changed && QS_RENDERER_REQUIRES_RESTART)
+                qs_append_restart_reason(restart_needed, restart_needed_size, "renderer");
+        }
+        if (parsed->video_options.resolution[0] != '\0') {
+            bool changed = qs_apply_resolution(parsed->video_options.resolution, live);
+            if (changed && QS_RESOLUTION_REQUIRES_RESTART)
+                qs_append_restart_reason(restart_needed, restart_needed_size, "resolution");
+        }
     }
-    if (parsed->video_options.resolution[0] != '\0') {
-        bool changed = qs_apply_resolution(parsed->video_options.resolution, live);
-        if (changed && QS_RESOLUTION_REQUIRES_RESTART)
-            qs_append_restart_reason(restart_needed, restart_needed_size, "resolution");
+    if (parsed->has_general) {
+        if (parsed->general.language[0] != '\0') {
+            bool changed = qs_apply_language(parsed->general.language, live);
+            if (changed && QS_LANGUAGE_REQUIRES_RESTART)
+                qs_append_restart_reason(restart_needed, restart_needed_size, "language");
+        }
     }
 }
 
@@ -507,6 +641,9 @@ static bool qs_config_write_file(void) {
     fprintf(f, "  \"video_options\": {\n");
     fprintf(f, "    \"renderer\": \"%s\",\n", qs_current_renderer());
     fprintf(f, "    \"resolution\": \"%s\"\n", qs_current_resolution());
+    fprintf(f, "  },\n");
+    fprintf(f, "  \"general\": {\n");
+    fprintf(f, "    \"language\": \"%s\"\n", qs_current_language_name());
     fprintf(f, "  }\n");
     fprintf(f, "}\n");
     fclose(f);
@@ -538,7 +675,10 @@ static jclass qs_find_class(JNIEnv *env, jobject activity, const char *name) {
     return result;
 }
 
-static void qs_show_restart_dialog(const char *restart_needed) {
+// Shared by qs_show_restart_dialog() and qs_show_message_dialog() below -
+// both call a MenuOverlay static method taking a single String and
+// returning void, differing only in which one.
+static void qs_call_dialog_method(const char *java_method_name, const char *message) {
     JNIEnv *env = (JNIEnv *)SDL_AndroidGetJNIEnv();
     jobject activity = (jobject)SDL_AndroidGetActivity();
     if (!env || !activity)
@@ -548,24 +688,34 @@ static void qs_show_restart_dialog(const char *restart_needed) {
     if (!menuOverlayClass)
         return;
 
-    jmethodID showRestartDialog =
-        (*env)->GetStaticMethodID(env, menuOverlayClass, "showRestartDialog", "(Ljava/lang/String;)V");
-    if (!showRestartDialog) {
+    jmethodID method =
+        (*env)->GetStaticMethodID(env, menuOverlayClass, java_method_name, "(Ljava/lang/String;)V");
+    if (!method) {
         (*env)->ExceptionClear(env);
         return;
     }
 
+    jstring messageStr = (*env)->NewStringUTF(env, message);
+    (*env)->CallStaticVoidMethod(env, menuOverlayClass, method, messageStr);
+    if ((*env)->ExceptionCheck(env))
+        (*env)->ExceptionClear(env);
+}
+
+static void qs_show_restart_dialog(const char *restart_needed) {
     char message[256];
     snprintf(message, sizeof(message),
              "The following changes require a restart to take effect: %s. Everything else has "
              "already been applied.",
              restart_needed);
-    jstring messageStr = (*env)->NewStringUTF(env, message);
-    (*env)->CallStaticVoidMethod(env, menuOverlayClass, showRestartDialog, messageStr);
-    if ((*env)->ExceptionCheck(env))
-        (*env)->ExceptionClear(env);
+    qs_call_dialog_method("showRestartDialog", message);
 }
 
+// Same restart-confirmation dialog view (MenuOverlay.java's
+// showMessageDialog()), minus its Restart Now button - for a plain
+// informational/warning message with nothing to restart into, currently
+// only qs_apply_language()'s missing-resource-files warning above.
+static void qs_show_message_dialog(const char *message) { qs_call_dialog_method("showMessageDialog", message); }
+
 // ===========================================================================
 // Public API (see qs_config.h)
 // ===========================================================================
@@ -574,19 +724,21 @@ static Uint32 s_save_event_type;
 static Uint32 s_load_event_type;
 static Uint32 s_set_renderer_event_type;
 static Uint32 s_set_resolution_event_type;
+static Uint32 s_set_language_event_type;
 
 void qs_config_load_startup(void) {
     // Registers the custom SDL event types qs_config_request_save()/_load()/
-    // _set_renderer()/_set_resolution() enqueue - done here, once, on the
-    // engine's own thread and before the menu/its buttons could possibly
-    // exist yet, rather than lazily from those functions themselves (which
-    // run on Android's UI thread and could otherwise race each other doing
-    // this lazily).
-    Uint32 base = SDL_RegisterEvents(4);
+    // _set_renderer()/_set_resolution()/_set_language() enqueue - done here,
+    // once, on the engine's own thread and before the menu/its buttons could
+    // possibly exist yet, rather than lazily from those functions themselves
+    // (which run on Android's UI thread and could otherwise race each other
+    // doing this lazily).
+    Uint32 base = SDL_RegisterEvents(5);
     s_save_event_type = base;
     s_load_event_type = base + 1;
     s_set_renderer_event_type = base + 2;
     s_set_resolution_event_type = base + 3;
+    s_set_language_event_type = base + 4;
 
     QsParsedConfig parsed;
     if (qs_config_read_file(&parsed))
@@ -623,6 +775,14 @@ void qs_config_request_set_resolution(int resolution_index) {
     SDL_PushEvent(&event);
 }
 
+void qs_config_request_set_language(int language_index) {
+    SDL_Event event;
+    SDL_zero(event);
+    event.type = s_set_language_event_type;
+    event.user.code = language_index;
+    SDL_PushEvent(&event);
+}
+
 bool qs_config_handle_event(const SDL_Event *event) {
     if (event->type == s_save_event_type) {
         if (qs_config_write_file())
@@ -655,5 +815,11 @@ bool qs_config_handle_event(const SDL_Event *event) {
             qs_apply_resolution(kResolutions[index].name, /*live=*/true);
         return true;
     }
+    if (event->type == s_set_language_event_type) {
+        int index = event->user.code;
+        if (index >= 0 && (size_t)index < QS_NUM_LANGUAGES)
+            qs_apply_language(kLanguages[index].name, /*live=*/true);
+        return true;
+    }
     return false;
 }