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)

CONFIG.md

@@ -51,6 +51,11 @@ startup, in addition to (and independent of) the engine's own `prefs.txt`.
   },
   "general": {
     "language": "english"
+  },
+  "player_position": {
+    "x": 0.0,
+    "z": 0.0,
+    "yaw_degrees": 0.0
   }
 }
 ```
@@ -94,3 +99,42 @@ project's own menu/keyboard chrome text (title bar, buttons, etc.).
   open. See README.md section 3 ("Game assets") for where `mfdfrn.res`/
   `mfdger.res` come from - the GOG Enhanced Edition installer alone doesn't
   include them.
+
+## Player Position (`player_position`)
+
+Where the player is virtually standing/facing, independent of - and not
+reset by - starting a new game or loading a save (same independence from
+`prefs.txt`/savegames as every other group here). Set by moving around
+in-headset (menu open, left thumbstick snap-turns, right thumbstick smooth-
+moves) to line yourself up with the game quad ("virtual screen"). Height is
+never included here - it always comes from your real tracked headset
+height, not this file.
+
+**Startup fallback**: only applied at startup if all three fields (`x`,
+`z`, `yaw_degrees`) are present - a config file with none of them (or none
+at all), or a partial/hand-edited one missing any of the three, falls back
+to an automatic recenter instead (see **Reset Position** below), using
+your real head pose at that moment. A complete saved position is applied
+as-is, with no such recenter.
+
+**Reset Position** (Main Menu tab) is a separate, live-only action, not a
+config value: it recenters using your *current* real head pose, so the
+game quad ends up directly ahead of wherever you're actually standing/
+facing right now - not a plain "go back to `(0, 0, 0)`" (which would only
+undo thumbstick movement, not any physical drift/rotation since the
+session started). It's unrelated to `questshock.config.json` - it never
+reads or writes this file.
+
+### `x`, `z`
+
+Horizontal position, in meters, on the play space's floor plane.
+
+- **Type**: number
+- **Restart required?**: No (live-appliable)
+
+### `yaw_degrees`
+
+Which way the player is virtually facing, in degrees.
+
+- **Type**: number
+- **Restart required?**: No (live-appliable)