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/laser_turret.h

@@ -0,0 +1,48 @@
+#ifndef JNR_LASER_TURRET_H
+#define JNR_LASER_TURRET_H
+
+#include "engine/entity.h"
+#include "engine/camera.h"
+#include "engine/tilemap.h"
+
+/* ═══════════════════════════════════════════════════
+ *  LASER TURRET — Hazard that charges up a visible
+ *  indicator beam, then fires a sustained laser for
+ *  several seconds. Two variants:
+ *    - Fixed direction (placed facing left/right)
+ *    - Tracking (slowly rotates toward the player
+ *      during idle/charge, locks on fire)
+ * ═══════════════════════════════════════════════════ */
+
+#define LASER_WIDTH         14
+#define LASER_HEIGHT        14
+#define LASER_CHARGE_TIME   1.5f   /* seconds to charge up         */
+#define LASER_FIRE_TIME     2.5f   /* seconds beam stays active    */
+#define LASER_COOLDOWN_TIME 2.0f   /* seconds before next cycle    */
+#define LASER_TRACK_SPEED   1.5f   /* radians/s tracking rate      */
+#define LASER_DAMAGE        1
+#define LASER_DAMAGE_CD     0.5f   /* seconds between damage ticks */
+#define LASER_HEALTH        3
+
+typedef enum LaserState {
+    LASER_IDLE,
+    LASER_CHARGING,
+    LASER_FIRING,
+    LASER_COOLDOWN,
+} LaserState;
+
+typedef struct LaserTurretData {
+    LaserState state;
+    float      timer;           /* countdown for current state     */
+    float      aim_angle;       /* current aim angle (radians)     */
+    float      damage_cd;       /* cooldown between damage ticks   */
+    bool       tracking;        /* true = slowly follows player    */
+    float      beam_end_x;     /* computed beam endpoint (world)  */
+    float      beam_end_y;
+} LaserTurretData;
+
+void    laser_turret_register(EntityManager *em);
+Entity *laser_turret_spawn(EntityManager *em, Vec2 pos);
+Entity *laser_turret_spawn_tracking(EntityManager *em, Vec2 pos);
+
+#endif /* JNR_LASER_TURRET_H */