Support large maps up to 8192x8192 tiles

Raise hard map size cap from 4096 to 8192 via MAX_MAP_SIZE constant.
Replace fixed 16 KB fgets buffer with dynamic line reader that handles
arbitrarily long tile rows. Check calloc returns for tile layer
allocation.

Add entity distance culling: skip updates for entities beyond 2x screen
width from camera, skip rendering beyond 64 px margin. Dead entities
bypass culling so cleanup callbacks always run. Player, spacecraft,
drone, and moving platforms set ENTITY_ALWAYS_UPDATE to opt out.

Replace naive 4-neighbor flood fill with scanline algorithm (O(height)
stack instead of O(area)) for safe use on large maps.

Raise MAX_ENTITY_SPAWNS from 128 to 512 and MAX_EXIT_ZONES from 8 to
16 to support populated large levels.

src/engine/entity.h

@@ -32,9 +32,10 @@ typedef enum EntityType {
 } EntityType;
 
 /* Entity flags */
-#define ENTITY_INVINCIBLE  (1 << 0)
-#define ENTITY_FACING_LEFT (1 << 1)
-#define ENTITY_DEAD        (1 << 2)
+#define ENTITY_INVINCIBLE    (1 << 0)
+#define ENTITY_FACING_LEFT   (1 << 1)
+#define ENTITY_DEAD          (1 << 2)
+#define ENTITY_ALWAYS_UPDATE (1 << 3)  /* never culled by distance   */
 
 typedef struct Entity {
     EntityType   type;
@@ -66,7 +67,8 @@ typedef struct EntityManager {
 void    entity_manager_init(EntityManager *em);
 Entity *entity_spawn(EntityManager *em, EntityType type, Vec2 pos);
 void    entity_destroy(EntityManager *em, Entity *e);
-void    entity_update_all(EntityManager *em, float dt, const Tilemap *map);
+void    entity_update_all(EntityManager *em, float dt, const Tilemap *map,
+                          const Camera *cam);
 void    entity_render_all(EntityManager *em, const Camera *cam);
 void    entity_manager_clear(EntityManager *em);