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_session.h

@@ -62,14 +62,14 @@ bool xr_frame_begin(void);
 // launcher and keyboard overlay quads via xr_overlay_render_and_build_layer()
 // for whichever of them is currently visible, and submits whichever of the
 // three quads actually rendered this frame as composition layers
-// positioned in front of the local reference space's origin, then ends the
-// XR frame.
+// positioned in front of the play space's origin (see
+// xr_session_set_player_transform() above), then ends the XR frame.
 void xr_frame_end(void);
 
 void xr_shutdown(void);
 
-// Local-space geometry of the single game quad xr_frame_end() submits
-// (distance in front of the local space origin, half-width/half-height,
+// Play-space geometry of the single game quad xr_frame_end() submits
+// (distance in front of the play space origin, half-width/half-height,
 // all in meters) - shared with xr_input.c's ray/quad hit-testing so the
 // laser pointer always matches whatever's actually visible.
 void xr_get_game_quad_extent(float *distance_m, float *half_width_m, float *half_height_m);
@@ -98,6 +98,49 @@ float xr_session_get_frame_ms(void);
 float xr_session_get_cpu_utilization(void);
 float xr_session_get_gpu_utilization(void);
 
+// The player's virtual position/orientation - horizontal-plane only (x/z
+// plus a yaw rotation about the vertical axis); height always comes from
+// the real tracked HMD pose, never from this. Everything the app submits
+// (game/menu/keyboard/beam quads) and every hand/head pose it locates is
+// expressed relative to a second reference space, derived from the real
+// LOCAL space by this transform's inverse (see xr_session.c's own
+// xr_session_rebuild_play_space()) - so changing it moves/turns the player
+// relative to those otherwise-fixed quads, letting them walk up to and
+// square up with the game quad ("virtual screen") without needing any
+// physical room-scale space to do it in.
+//
+// xr_session_set_player_transform() only stores the three floats (no XR
+// calls) - safe to call before xr_init() has even run, which is how
+// qs_config_load_startup() seeds the saved position before the OpenXR
+// session/spaces exist yet (xr_init() picks it up when it creates its own
+// spaces); also used live by xr_input.c's thumbstick locomotion and
+// "Reset Position" recenter (xr_input_recenter_player_to_screen()) and
+// qs_config.c's "Load Configuration", and read back by "Save Configuration"
+// via the getter.
+void xr_session_set_player_transform(float x, float z, float yaw_deg);
+void xr_session_get_player_transform(float *x, float *z, float *yaw_deg);
+
+// The real, physical tracking-origin reference space - used only by
+// xr_input_recenter_player_to_screen() (xr_input.c), which needs the
+// player's CURRENT real head pose (not one already bent through the
+// existing play-space transform - see xr_session_set_player_transform()'s
+// own comment above) to compute a brand new one. Every other consumer in
+// this app (rendering, hit-testing) uses the play space instead - see
+// xr_session_rebuild_play_space()'s own comment for why this one stays a
+// separate, never-swapped-out space.
+XrSpace xr_session_get_local_space(void);
+
+// The most recent xrWaitFrame-predicted display time (xr_frame_begin()'s
+// own g_predicted_display_time) - for the same reason as
+// xr_session_get_local_space() above, xr_input_recenter_player_to_screen()
+// needs a valid XrTime to locate a space with but isn't itself called from
+// the normal per-frame xr_frame_end()/xr_input_sync_and_draw() call chain
+// that already threads one through (it's triggered by a button press,
+// dispatched via qs_config.c's cross-thread event queue instead). A frame
+// or so of staleness doesn't matter for a one-shot recenter. 0 before the
+// first frame has ever begun.
+XrTime xr_session_get_predicted_display_time(void);
+
 #ifdef __cplusplus
 }
 #endif