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/editor.h

@@ -26,6 +26,7 @@ typedef enum EditorTool {
     TOOL_FILL,          /* flood fill area                */
     TOOL_ENTITY,        /* place/select entities          */
     TOOL_SPAWN,         /* set player spawn point         */
+    TOOL_EXIT,          /* place/delete exit zones        */
     TOOL_COUNT
 } EditorTool;
 
@@ -37,9 +38,9 @@ typedef enum EditorLayer {
 } EditorLayer;
 
 /* UI panel regions */
-#define EDITOR_TOOLBAR_H   14   /* top bar height                  */
+#define EDITOR_TOOLBAR_H   18   /* top bar height                  */
 #define EDITOR_PALETTE_W   80   /* right panel width               */
-#define EDITOR_STATUS_H    10   /* bottom status bar height        */
+#define EDITOR_STATUS_H    16   /* bottom status bar height        */
 
 typedef struct Editor {
     /* ── Map data ──────────────────────────── */