Fix laser beam blocked by own tile, consolidate enemy-type checks

Beam raycast now skips the tile the turret is mounted on, so
wall-embedded laser turrets can shoot outward.

Extract entity_is_enemy() into engine/entity.c as single source
of truth for enemy-type checks. Replaces triplicated type lists
in level.c, drone.c, and projectile.c that caused the collision
bug when new enemy types were added.

src/engine/entity.c

@@ -126,3 +126,17 @@ void entity_register(EntityManager *em, EntityType type,
     em->render_fn[type] = render;
     em->destroy_fn[type] = destroy;
 }
+
+bool entity_is_enemy(const Entity *e) {
+    switch (e->type) {
+        case ENT_ENEMY_GRUNT:
+        case ENT_ENEMY_FLYER:
+        case ENT_TURRET:
+        case ENT_ENEMY_CHARGER:
+        case ENT_SPAWNER:
+        case ENT_LASER_TURRET:
+            return true;
+        default:
+            return false;
+    }
+}

src/engine/entity.h

@@ -80,4 +80,8 @@ void entity_register(EntityManager *em, EntityType type,
                      EntityUpdateFn update, EntityRenderFn render,
                      EntityDestroyFn destroy);
 
+/* Returns true if the entity is a hostile enemy type.
+ * Single source of truth — add new enemy types here. */
+bool entity_is_enemy(const Entity *e);
+
 #endif /* JNR_ENTITY_H */