Add moon surface intro level with asteroid hazards and unarmed mechanics

Introduce moon01.lvl as the starting level — a pure jump-and-run intro
with no gun and no enemies, just platforming over gaps and dodging falling
asteroids. The player picks up their gun upon transitioning to level01.

New features:
- Moon tileset and PARALLAX_STYLE_MOON with crater terrain backgrounds
- Asteroid entity (ENT_ASTEROID): falls from sky, damages on contact,
  explodes on ground with particles, respawns after delay
- PLAYER_UNARMED directive disables gun for the level
- Pit rescue mechanic: falling costs 1 HP and auto-dashes upward
- Gun powerup entity type for future armed-pickup levels
- Segment-based procedural level generator with themed rooms
- Extended editor with entity palette and improved tile cycling
- Web shell improvements for Emscripten builds

include/config.h

@@ -22,7 +22,10 @@
 
 /* ── Entities ───────────────────────────────────────── */
 #define MAX_ENTITIES       512
-#define MAX_ENTITY_SPAWNS  64     /* max entity spawns per level    */
+#define MAX_ENTITY_SPAWNS  128    /* max entity spawns per level    */
+
+/* ── Level transitions ─────────────────────────────── */
+#define MAX_EXIT_ZONES     8      /* max exit zones per level       */
 
 /* ── Rendering ──────────────────────────────────────── */
 #define MAX_SPRITES    2048       /* max queued sprites per frame   */