Add hand-built on-screen keyboard to the OpenXR menu quad

MenuOverlay's key grid forwards every press to the game as real input:
non-printable keys (Esc/Enter/Backspace) as an SDLActivity.onNativeKeyDown()/
onNativeKeyUp() pair, and printable keys (letters, Space) as a synthesized
SDL_TEXTINPUT event pushed directly from native code, since the engine's
pump_events() only picks up printable ASCII from that event type, not from
SDL_KEYDOWN. Confirmed on-device, including that Space now correctly skips
the intro cutscene alongside Esc/Enter.

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

@@ -7,9 +7,32 @@
 #include <jni.h>
 #include <unistd.h>
 
+#include <SDL.h>
+
 JNIEXPORT void JNICALL
 Java_de_ladkau_questshock_QuestShockActivity_nativeChdir(JNIEnv *env, jclass clazz, jstring path) {
     const char *cpath = (*env)->GetStringUTFChars(env, path, NULL);
     chdir(cpath);
     (*env)->ReleaseStringUTFChars(env, path, cpath);
 }
+
+// MenuOverlay's hand-built on-screen keyboard (see
+// MenuOverlay.commitPrintableChar()) has no real Android IME session behind
+// it, so printable characters can't reach the game via Android's usual IME
+// path - instead this builds and pushes an SDL_TEXTINPUT event directly,
+// the same event type/shape SDL's own Android backend pushes for typed
+// text, using only the public SDL_Event/SDL_PushEvent API. The engine's
+// pump_events() (sdl_events.c) only picks up printable characters from
+// this event, not from SDL_KEYDOWN (see handleKeyPress() in MenuOverlay.java
+// for why).
+JNIEXPORT void JNICALL
+Java_de_ladkau_questshock_MenuOverlay_nativeSendPrintableChar(JNIEnv *env, jclass clazz, jchar c) {
+    SDL_Event event;
+    SDL_zero(event);
+    event.type = SDL_TEXTINPUT;
+    event.text.timestamp = SDL_GetTicks();
+    event.text.windowID = 0;
+    event.text.text[0] = (char)c;
+    event.text.text[1] = '\0';
+    SDL_PushEvent(&event);
+}

android/app/src/main/cpp/xr_menu.h

@@ -1,8 +1,8 @@
 // Menu quad for questshock's immersive Quest build: MenuOverlay.java's
-// off-screen-rendered View tree (currently just a "Keyboard" button - see
-// the plan's step D for the on-screen keyboard that goes behind it),
-// toggled by xr_input.c's menu_toggle action and hit-tested by the same ray
-// xr_input.c already computes per hand.
+// off-screen-rendered View tree (a "Keyboard" button that swaps to a
+// hand-built on-screen key grid - there's no system IME to borrow once
+// immersive), toggled by xr_input.c's menu_toggle action and hit-tested by
+// the same ray xr_input.c already computes per hand.
 //
 // The menu has its own independent OpenXR swapchain, sized to exactly
 // xr_menu_get_content_size()'s dimensions (see xr_session.c, which creates