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/game/hazards.h

@@ -87,4 +87,28 @@ typedef struct ForceFieldData {
 void    force_field_register(EntityManager *em);
 Entity *force_field_spawn(EntityManager *em, Vec2 pos);
 
+/* ═══════════════════════════════════════════════════
+ *  ASTEROID — Falls from the sky, damages the player
+ *  on contact, explodes on hitting the ground, then
+ *  respawns at its original position after a delay.
+ * ═══════════════════════════════════════════════════ */
+
+#define ASTEROID_WIDTH     14
+#define ASTEROID_HEIGHT    14
+#define ASTEROID_FALL_SPEED 120.0f   /* base fall speed (px/s)       */
+#define ASTEROID_DAMAGE    1
+#define ASTEROID_RESPAWN   3.0f      /* seconds before respawning    */
+
+typedef struct AsteroidData {
+    Vec2  spawn_pos;                 /* original position (for reset)*/
+    float fall_speed;                /* current fall velocity        */
+    float respawn_timer;             /* countdown while inactive     */
+    bool  falling;                   /* true = falling, false = wait */
+    float trail_timer;               /* particle trail interval      */
+    float start_delay;               /* initial delay before first fall */
+} AsteroidData;
+
+void    asteroid_register(EntityManager *em);
+Entity *asteroid_spawn(EntityManager *em, Vec2 pos);
+
 #endif /* JNR_HAZARDS_H */