Add pause menu, laser turret, charger/spawner enemies, and Mars campaign

Implement four feature phases:

Phase 1 - Pause menu: extract bitmap font into shared engine/font
module, add MODE_PAUSED with Resume/Restart/Quit overlay.

Phase 2 - Laser turret hazard: ENT_LASER_TURRET with charge/fire/
cooldown state machine, per-pixel beam raycast, two variants (fixed
and tracking). Registered in entity registry with editor icons.

Phase 3 - Charger and Spawner enemies: charger ground patrol with
detect/telegraph/charge/stun cycle (2s charge timeout), spawner that
periodically creates grunts up to a global cap of 3.

Phase 4 - Mars campaign: two handcrafted levels (mars01 surface,
mars02 base), mars_tileset.png, PARALLAX_STYLE_MARS with salmon sky
and red mesas, THEME_MARS_SURFACE/THEME_MARS_BASE for the procedural
generator with per-theme gravity/tileset/parallax. Moon campaign now
chains moon03 -> mars01 -> mars02 -> victory.

Also fix review findings: deterministic seed on generated level
restart, NULL checks on calloc in spawn functions, charge timeout
to prevent infinite charge on flat terrain, and stop suppressing
stderr in Makefile web-serve target so real errors are visible.

src/game/levelgen.h

@@ -23,6 +23,10 @@ typedef enum LevelTheme {
                               force fields, turrets, medium gravity       */
     THEME_SPACE_STATION,   /* orbital station: shafts, platforms, low
                               gravity, moving platforms, turrets          */
+    THEME_MARS_SURFACE,    /* Mars exterior: red dusty terrain, wind,
+                              wide-open with charger enemies             */
+    THEME_MARS_BASE,       /* Mars base interior: very vertical, narrow
+                              corridors, turrets, laser turrets, spawners*/
     THEME_COUNT
 } LevelTheme;