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

DESIGN.md

@@ -76,9 +76,9 @@ Already implemented: `GRAVITY`, `BG_COLOR`, `MUSIC`, `PARALLAX_FAR`, `PARALLAX_N
 ### Implemented
 - **Grunt** — Red spiky ground patrol. Walks back and forth, turns at edges/walls. 2 HP.
 - **Flyer** — Purple bat-like. Bobs in air, chases player when close, shoots fireballs. 1 HP.
+- **Turret** — Stationary, rotates to aim at player, fires periodically
 
 ### Planned
-- **Turret** — Stationary, rotates to aim at player, fires periodically
 - **Charger** — Detects player at range, charges in a straight line at high speed
 - **Shielder** — Has a directional shield, must be hit from behind or above
 - **Spawner** — Stationary, periodically spawns smaller enemies
@@ -125,12 +125,16 @@ Current directives: `TILESET`, `SIZE`, `SPAWN`, `GRAVITY`, `BG_COLOR`, `MUSIC`,
 - `WIND`, `STORM`, `DRAG` — Atmosphere settings
 
 ### Level Ideas
-1. **Derelict Station** — Low gravity, dark, flickering lights, abandoned corridors
-2. **Desert Planet** — High gravity, sand storm wind, bright orange palette
-3. **Gas Giant Moon** — Very low gravity, floating platforms, toxic atmosphere
-4. **Asteroid Belt** — Zero-G sections, small disconnected platforms
-5. **Space Freighter** — Normal gravity, tight corridors, turret enemies
-6. **Volcanic World** — Normal gravity, rising lava hazard, fire enemies
+
+0. **Intro Level (Moon)** - Low gravity, bright surface, spacey/no obstacles
+1. **Mars Surface** - Low gravity, red surface, spacey, little to no obstacles, transition area is entry to base
+2. **Mars Base** - Normal gravity, very vertical, narrow, 90 degree turns, lots of enemies 
+3. **Derelict Station** — Low gravity, dark, flickering lights, abandoned corridors
+4. **Desert Planet** — High gravity, sand storm wind, bright orange palette
+5. **Gas Giant Moon** — Very low gravity, floating platforms, toxic atmosphere
+6. **Asteroid Belt** — Zero-G sections, small disconnected platforms
+7. **Space Freighter** — Normal gravity, tight corridors, turret enemies
+8. **Volcanic World** — Normal gravity, rising lava hazard, fire enemies
 
 ---
 
@@ -153,21 +157,21 @@ Current directives: `TILESET`, `SIZE`, `SPAWN`, `GRAVITY`, `BG_COLOR`, `MUSIC`,
 
 ### High Priority
 - [x] Entity spawn directives in .lvl format (`ENTITY` directive)
-- [ ] Level exit zones and level transitions
+- [x] Level exit zones and level transitions
 - [x] Dash mechanic
-- [ ] Wall slide / wall jump
 - [x] Particle system (death puffs, landing dust, projectile impact sparks, wall slide dust)
 - [x] Screen shake on damage / enemy kills
 - [x] Sound effects (jump, shoot, hit, enemy death, dash)
 - [x] Basic HUD (health hearts + jetpack charges)
 
 ### Medium Priority
+- [x] In-game level editor (tile/entity placement, save/load, test play)
 - [ ] Wind / drag atmosphere properties
 - [x] Parallax scrolling backgrounds (procedural stars + nebula, or from image files)
 - [x] Per-level background color (`BG_COLOR` directive)
 - [x] Music playback per level (`MUSIC` directive)
 - [ ] Weapon switching system
-- [ ] Pickup entities (health, ammo, credits)
+- [x] Pickup entities (health, jetpack refill, drone companion)
 - [ ] Better tileset art (space-themed)
 - [ ] Player sprite polish (more animation frames)
 - [x] Death / respawn system
@@ -212,6 +216,5 @@ Current directives: `TILESET`, `SIZE`, `SPAWN`, `GRAVITY`, `BG_COLOR`, `MUSIC`,
 ## Reference Games
 - Jazz Jackrabbit 2 (movement feel, weapon variety, level design)
 - Metal Slug (run-and-gun, enemy variety, visual flair)
-- Mega Man X (wall jump, dash, tight controls)
-- Cave Story (atmosphere, exploration, story integration)
+- Mega Man X (dash, tight controls)
 - Cowboy Bebop (aesthetic, tone, music style)