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.

TODO.md

@@ -94,14 +94,47 @@ Implemented in `enemy.h`/`enemy.c`:
   color scheme. Both registered in entity registry with editor icons.
 
 ## ~~Mars campaign~~ ✓
-Implemented: two handcrafted levels plus procedural generator support.
+Implemented: three handcrafted levels, a dedicated Mars Base generator, and
+Mars themes in all generic segment generators.
+
+### Handcrafted levels
 - **mars01.lvl** (250×23, Mars Surface): low gravity (370), wind, wide-open
   red terrain, charger + grunt enemies, spacecraft intro. New
   `mars_tileset.png` and `PARALLAX_STYLE_MARS` (salmon sky, red mesas, dust).
 - **mars02.lvl** (40×46, Mars Base): normal gravity (700), tall vertical
   corridors with narrow passages, turrets, laser turrets (fixed + tracking),
-  spawners, chargers, grunts. Victory exit at bottom.
-- Generator: `THEME_MARS_SURFACE` / `THEME_MARS_BASE` with per-theme gravity,
-  bg color, parallax style, tileset path, segment probabilities, and height
-  zone assignment. Mars themes added to procedural progression (6 options).
-- Moon campaign now chains to Mars: moon03 → mars01 → mars02 → victory.
+  spawners, chargers, grunts. Exits to `generate:mars_base`.
+- **mars03.lvl** (30×23, Boss Arena): heavy enemies (spawners, chargers,
+  laser turrets), exits to `generate:station`.
+
+### Mars Base dedicated generator
+- `levelgen_generate_mars_base()` with 6 custom segment types: Entry (safe
+  start), Shaft (vertical with alternating platforms + laser turrets),
+  Corridor (narrow horizontal passage), Turret Hall (three-level laser
+  gauntlet), Hive (spawner room), Arena (tall multi-level combat).
+- 46-tile tall levels, Mars tileset, interior parallax, 700 gravity.
+- Depth scaling: 6→7→8 segments, difficulty 0.5→0.7→0.9.
+- After 2 generated levels, exit points to mars03.lvl (boss arena).
+- `generate:mars_base` exit target handling in main.c with `s_mars_depth`.
+
+### Mars themes in generic generators
+- All 7 generic segment generators (`gen_flat`, `gen_pit`, `gen_platforms`,
+  `gen_corridor`, `gen_arena`, `gen_shaft`, `gen_transition`) place
+  Mars-specific enemies (chargers, laser turrets, spawners) when the active
+  theme is `THEME_MARS_SURFACE` or `THEME_MARS_BASE`.
+
+### Music
+- `kaffe_og_kage.ogg` used for all Mars levels (handcrafted + both generators).
+
+### Level chain
+- moon03 → mars01 → mars02 → generate:mars_base (×2) → mars03 → generate:station.
+
+### Bug fixes (this batch)
+- Deterministic seed on generated level restart (`s_gen_seed` snapshot).
+- NULL checks on calloc in `charger_spawn`, `spawner_spawn`, `laser_spawn_internal`.
+- Charger charge timeout (2 s max) prevents infinite charge on flat terrain.
+- Removed no-op `flags |= 0` in spawner_spawn.
+- Jetpack fuel pickup preserves recharge progress instead of resetting timer.
+- Makefile web-serve: removed `2>/dev/null` so real errors are visible.
+- `s_mars_depth` and `s_station_depth` reset when game loops back to beginning.
+- Added `gen_bg_decoration()` call to Mars Base generator.