Add configurable game/initial screen width and distance, with a bundled first-run config

- Video Options tab offers width and distance presets independently for
  the Initial Screen (splash/intro/menu) and Game Screen (gameplay)
  profiles, with their preset step/offset configurable via
  questshock.config.json.
- Ships res/questshock.config.default.json as the initial config on
  first run (or whenever questshock.config.json doesn't exist yet),
  refreshed from the APK on every launch without touching a user's own
  saved config.

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

@@ -68,11 +68,30 @@ void xr_frame_end(void);
 
 void xr_shutdown(void);
 
-// Play-space geometry of the single game quad xr_frame_end() submits
+// Play-space geometry of the single game screen 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);
+// laser pointer always matches whatever's actually visible. *radius_m/
+// *central_angle_rad are 0 when flat, in which case *half_width_m is the
+// horizontal extent; otherwise the screen is a curved cylinder segment
+// and the caller uses *central_angle_rad instead of *half_width_m for the
+// horizontal bound (*half_height_m stays meaningful either way). Curving
+// is currently unused - qs_config.c has no config surface for it - but
+// kept working so it can be re-enabled later.
+void xr_get_game_quad_extent(float *distance_m, float *half_width_m, float *half_height_m,
+                              float *radius_m, float *central_angle_rad);
+
+// Sets the game screen's current physical width (meters) and curve (wrap
+// angle in degrees, 0 = flat) - a plain state setter, no XR calls, same
+// category as xr_session_set_player_transform() above (safe to call
+// before xr_init() has even run). Read by both xr_frame_end() (to build/
+// submit the actual layer) and, via xr_get_game_quad_extent() above,
+// xr_input.c's hit-testing. Driven by qs_config.c's
+// qs_screen_profile_notify_loop_mode(), which decides which of the
+// "Initial Screen"/"Game Screen" config profiles (see CONFIG.md) is
+// currently active - this function itself has no notion of "profiles",
+// only "what shape is current".
+void xr_session_set_game_screen_shape(float width_m, float curve_deg, float distance_m);
 
 // The engine's current screen resolution (gScreenWide/gScreenHigh,
 // InitMac.h) - not the game quad's own swapchain size, which is fixed at