Add VR locomotion: move/turn with thumbsticks while the menu is open

- Left thumbstick snap-turns, right thumbstick smooth-moves, letting the
  player line themselves up with the fixed game quad ("virtual screen")
  without needing physical room-scale space to do it in. Implemented via
  a second OpenXR reference space (a "play space") derived from the real
  LOCAL space by the inverse of the player's transform, so every existing
  quad/hit-testing/rendering call site just needed g_local_space swapped
  for it - no manual coordinate-transform math required elsewhere
- Generalized the ray/quad hit-testing in xr_input.c to a proper
  oriented-plane intersection (needed once quads can rotate); verified it
  degenerates to the old axis-aligned formula when unrotated, so the
  keyboard and non-recentered menu are unaffected
- The menu launcher now re-centers in front of the player, correctly
  oriented to face them, each time it's freshly reopened
- "Reset Position" (Main Menu tab) recenters using the player's current
  real head pose - not a plain reset-to-origin, which only undid
  thumbstick movement and ignored physical drift since the session
  started. The same recenter runs once automatically at startup too, as a
  fallback for when there's no complete saved position to use instead
- Position/orientation persists in questshock.config.json's new
  player_position group (independent of savegames), including new
  numeric-value support in the config file's hand-rolled JSON parser
  (previously string-enums only)

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

@@ -1,6 +1,7 @@
 #include "xr_overlay.h"
 
 #include <dlfcn.h>
+#include <math.h>
 #include <stdlib.h>
 #include <string.h>
 
@@ -18,11 +19,33 @@
 #define LOGE(...) __android_log_print(ANDROID_LOG_ERROR, TAG, __VA_ARGS__)
 #define LOGI(...) __android_log_print(ANDROID_LOG_INFO, TAG, __VA_ARGS__)
 
+// Same constant (deliberately not <math.h>'s M_PI, which isn't used
+// anywhere else in this codebase and isn't guaranteed by the C standard -
+// only POSIX) as xr_session.c's own play-space rebuild math.
+#define QS_DEG_TO_RAD (3.14159265358979323846f / 180.0f)
+
 struct XrOverlay {
     XrInstance instance; // for error logging only
     int width, height;
     float distance_m, half_width_m, half_height_m;
     float center_x_m, center_y_m;
+    // The *authored* distance_m (config->distance_m at creation time),
+    // never touched again afterward by anything, including
+    // xr_overlay_set_pose() - kept separate from the live, mutable
+    // distance_m above because that one stops meaning "how far in front of
+    // the origin" once a rotated set_pose() call (the menu recenter)
+    // repurposes it as a raw base-space Z coordinate; reusing *that* as a
+    // "how far in front of me" magnitude on a later recenter would drift
+    // further with every reopen instead of staying at a stable distance -
+    // see xr_overlay_get_default_distance_m().
+    float default_distance_m;
+    // Rotation about the vertical axis, applied on top of center_x_m/
+    // center_y_m/distance_m (see xr_overlay_set_pose()) - 0 for every
+    // overlay except the menu launcher immediately after it's freshly
+    // reopened (xr_input.c re-centers it in front of wherever the player
+    // is currently facing, which needs this to keep the quad actually
+    // facing them instead of whatever direction it started out at).
+    float yaw_deg;
     bool visible;
 
     XrSwapchainState swapchain;
@@ -205,6 +228,7 @@ XrOverlay *xr_overlay_create(XrInstance instance, XrSession session, int64_t swa
     overlay->width = config->width;
     overlay->height = config->height;
     overlay->distance_m = config->distance_m;
+    overlay->default_distance_m = config->distance_m;
     overlay->half_width_m = config->width_m * 0.5f;
     overlay->half_height_m = config->width_m * 0.5f * (float)config->height / (float)config->width;
     overlay->center_x_m = config->default_center_x_m;
@@ -281,9 +305,10 @@ void xr_overlay_set_visible(XrOverlay *overlay, bool visible) {
 bool xr_overlay_is_visible(const XrOverlay *overlay) { return overlay != NULL && overlay->visible; }
 
 void xr_overlay_get_quad_extent(const XrOverlay *overlay, float *center_x_m, float *center_y_m,
-                                 float *distance_m, float *half_width_m, float *half_height_m) {
+                                 float *distance_m, float *half_width_m, float *half_height_m,
+                                 float *yaw_deg) {
     if (overlay == NULL) {
-        *center_x_m = *center_y_m = *distance_m = *half_width_m = *half_height_m = 0.0f;
+        *center_x_m = *center_y_m = *distance_m = *half_width_m = *half_height_m = *yaw_deg = 0.0f;
         return;
     }
     *center_x_m = overlay->center_x_m;
@@ -291,6 +316,7 @@ void xr_overlay_get_quad_extent(const XrOverlay *overlay, float *center_x_m, flo
     *distance_m = overlay->distance_m;
     *half_width_m = overlay->half_width_m;
     *half_height_m = overlay->half_height_m;
+    *yaw_deg = overlay->yaw_deg;
 }
 
 void xr_overlay_set_position(XrOverlay *overlay, float center_x_m, float center_y_m) {
@@ -300,6 +326,20 @@ void xr_overlay_set_position(XrOverlay *overlay, float center_x_m, float center_
     overlay->center_y_m = center_y_m;
 }
 
+void xr_overlay_set_pose(XrOverlay *overlay, float center_x_m, float center_y_m, float distance_m,
+                          float yaw_deg) {
+    if (overlay == NULL)
+        return;
+    overlay->center_x_m = center_x_m;
+    overlay->center_y_m = center_y_m;
+    overlay->distance_m = distance_m;
+    overlay->yaw_deg = yaw_deg;
+}
+
+float xr_overlay_get_default_distance_m(const XrOverlay *overlay) {
+    return overlay == NULL ? 0.0f : overlay->default_distance_m;
+}
+
 void xr_overlay_touch(XrOverlay *overlay, float u, float v, bool down) {
     if (overlay == NULL)
         return;
@@ -443,7 +483,17 @@ bool xr_overlay_render_and_build_layer(XrOverlay *overlay, XrCompositionLayerQua
     out_quad->subImage.swapchain = overlay->swapchain.swapchain;
     out_quad->subImage.imageRect.extent.width = overlay->swapchain.width;
     out_quad->subImage.imageRect.extent.height = overlay->swapchain.height;
-    out_quad->pose.orientation.w = 1.0f;
+    // Rotation about the vertical axis only (see the struct field's own
+    // comment) - same R_y(-yaw_deg) quaternion construction as
+    // xr_session.c's own play-space rebuild, for the same reason: this
+    // quad's pose is expressed as "yaw_deg turned from its own local -Z",
+    // so a point/ray in that rotated frame maps back into the (unrotated)
+    // base space by rotating by -yaw_deg.
+    float half = -(overlay->yaw_deg * QS_DEG_TO_RAD) * 0.5f;
+    out_quad->pose.orientation.x = 0.0f;
+    out_quad->pose.orientation.y = sinf(half);
+    out_quad->pose.orientation.z = 0.0f;
+    out_quad->pose.orientation.w = cosf(half);
     out_quad->pose.position.x = overlay->center_x_m;
     out_quad->pose.position.y = overlay->center_y_m;
     out_quad->pose.position.z = -overlay->distance_m;