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

src/engine/tilemap.h

@@ -30,6 +30,14 @@ typedef struct EntitySpawn {
     float    x, y;             /* world position (pixels)        */
 } EntitySpawn;
 
+/* Exit zone — triggers level transition on player overlap */
+typedef struct ExitZone {
+    float    x, y, w, h;                    /* world-space bounding box    */
+    char     target[ASSET_PATH_MAX];        /* path to next level, or
+                                             * "generate" for procgen,
+                                             * or empty for "you win"      */
+} ExitZone;
+
 typedef struct Tilemap {
     int          width, height;     /* map size in tiles          */
     uint16_t    *bg_layer;          /* background tile IDs        */
@@ -47,8 +55,11 @@ typedef struct Tilemap {
     char         parallax_far_path[ASSET_PATH_MAX];  /* far bg image path     */
     char         parallax_near_path[ASSET_PATH_MAX]; /* near bg image path    */
     int          parallax_style;    /* procedural bg style (0=default)         */
+    bool         player_unarmed;   /* if true, player starts without gun       */
     EntitySpawn  entity_spawns[MAX_ENTITY_SPAWNS];
     int          entity_spawn_count;
+    ExitZone     exit_zones[MAX_EXIT_ZONES];
+    int          exit_zone_count;
 } Tilemap;
 
 bool     tilemap_load(Tilemap *map, const char *path, SDL_Renderer *renderer);