Add Mars Base generator, mars03 boss arena, and Mars campaign polish

Dedicated Mars Base generator with 6 segment types (entry, shaft, corridor,
turret hall, hive, arena), depth-scaled difficulty, and exit to boss arena
after 2 generated levels. Mars themes integrated into all 7 generic segment
generators. Full level chain: moon03 → mars01 → mars02 → mars_base (×2) →
mars03 → station.

Also: jetpack fuel pickup preserves recharge progress, depth counters reset
on game loop, NULL checks on entity spawns, charger charge timeout, bg
decoration in Mars Base generator.

DESIGN.md

@@ -78,10 +78,12 @@ Already implemented: `GRAVITY`, `WIND`, `BG_COLOR`, `MUSIC`, `PARALLAX_FAR`, `PA
 - **Flyer** — Purple bat-like. Bobs in air, chases player when close, shoots fireballs. 1 HP.
 - **Turret** — Stationary, rotates to aim at player, fires periodically
 
+- **Charger** — Ground patrol, detects player in 200 px horizontal LOS, ALERT → CHARGE (150 px/s) → STUNNED on wall hit. 2 HP.
+- **Spawner** — Stationary, spawns grunts every 4.5 s (max 3 alive). 3 HP, destructible.
+- **Laser Turret** — State machine (IDLE → CHARGING → FIRING → COOLDOWN). Per-pixel beam raycast. Fixed variant aims left; tracking variant rotates toward player at 1.5 rad/s.
+
 ### Planned
-- **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
 - **Boss** — Large, multi-phase encounters. One per world area.
 
 ---
@@ -175,7 +177,7 @@ Current directives: `TILESET`, `SIZE`, `SPAWN`, `GRAVITY`, `WIND`, `BG_COLOR`, `
 - [ ] Better tileset art (space-themed)
 - [ ] Player sprite polish (more animation frames)
 - [x] Death / respawn system
-- [ ] Pause menu
+- [x] Pause menu
 
 ### Low Priority (Future)
 - [ ] World map mode
@@ -209,7 +211,7 @@ Current directives: `TILESET`, `SIZE`, `SPAWN`, `GRAVITY`, `WIND`, `BG_COLOR`, `
 | Aim diag  | UP+LEFT/RIGHT (+ shoot) | Implemented |
 | Dash      | C            | Implemented   |
 | Look up   | UP (stand still) | Implemented |
-| Pause     | Escape       | Quits game    |
+| Pause     | Escape       | Implemented   |
 
 ---