Improve enemy AI cliff detection, add level-bottom kill, and charger knockback

Enemies that fall past the bottom of the level are now instantly destroyed,
preventing them from accumulating off-screen. Cliff detection uses a
speed-scaled lookahead so faster enemies reverse earlier. Charger deals
double damage during charge and knockback scales with the attacker's speed.

src/game/enemy.c

@@ -5,9 +5,46 @@
 #include "engine/renderer.h"
 #include "engine/particle.h"
 #include "engine/audio.h"
+#include "config.h"
 #include <stdlib.h>
 #include <math.h>
 
+/* ── Shared helpers ───────────────────────────────── */
+
+/* Kill enemy if it fell past the bottom of the level. */
+static bool enemy_check_level_bottom(Entity *self, const Tilemap *map,
+                                      EntityManager *em) {
+    float level_bottom = (float)(map->height * TILE_SIZE);
+    if (self->body.pos.y > level_bottom) {
+        self->flags |= ENTITY_DEAD;
+        self->health = 0;
+        entity_destroy(em, self);
+        return true;
+    }
+    return false;
+}
+
+/* Check for cliff ahead of a ground enemy. Returns true if cliff detected.
+ * Uses a wider lookahead that scales with speed for fast-moving enemies. */
+static bool enemy_detect_cliff(const Body *body, float patrol_dir,
+                                float speed, const Tilemap *map) {
+    if (!body->on_ground) return false;
+
+    /* Scale lookahead with speed: at least 4px, up to speed/10 */
+    float lookahead = speed * 0.1f;
+    if (lookahead < 4.0f) lookahead = 4.0f;
+
+    float check_x = (patrol_dir > 0) ?
+        body->pos.x + body->size.x + lookahead :
+        body->pos.x - lookahead;
+    float check_y = body->pos.y + body->size.y + 4.0f;
+
+    int tx = world_to_tile(check_x);
+    int ty = world_to_tile(check_y);
+
+    return !tilemap_is_solid(map, tx, ty);
+}
+
 /* ════════════════════════════════════════════════════
  *  GRUNT - ground patrol enemy
  * ════════════════════════════════════════════════════ */
@@ -20,6 +57,9 @@ static void grunt_update(Entity *self, float dt, const Tilemap *map) {
 
     Body *body = &self->body;
 
+    /* Kill if fallen off bottom of level */
+    if (enemy_check_level_bottom(self, map, s_grunt_em)) return;
+
     /* Death sequence */
     if (self->flags & ENTITY_DEAD) {
         animation_set(&self->anim, &anim_grunt_death);
@@ -48,19 +88,10 @@ static void grunt_update(Entity *self, float dt, const Tilemap *map) {
         gd->patrol_dir = -gd->patrol_dir;
     }
 
-    /* Turn around at ledge: check if there's ground ahead */
-    if (body->on_ground) {
-        float check_x = (gd->patrol_dir > 0) ?
-            body->pos.x + body->size.x + 2.0f :
-            body->pos.x - 2.0f;
-        float check_y = body->pos.y + body->size.y + 4.0f;
-
-        int tx = world_to_tile(check_x);
-        int ty = world_to_tile(check_y);
-
-        if (!tilemap_is_solid(map, tx, ty)) {
-            gd->patrol_dir = -gd->patrol_dir;
-        }
+    /* Turn around at ledge */
+    if (enemy_detect_cliff(body, gd->patrol_dir, GRUNT_SPEED, map)) {
+        gd->patrol_dir = -gd->patrol_dir;
+        body->vel.x = 0;
     }
 
     /* Animation */
@@ -146,12 +177,14 @@ static Entity *find_player(EntityManager *em) {
 }
 
 static void flyer_update(Entity *self, float dt, const Tilemap *map) {
-    (void)map;  /* flyers don't collide with tiles */
     FlyerData *fd = (FlyerData *)self->data;
     if (!fd) return;
 
     Body *body = &self->body;
 
+    /* Kill if fallen off bottom of level */
+    if (enemy_check_level_bottom(self, map, s_flyer_em)) return;
+
     /* Death sequence */
     if (self->flags & ENTITY_DEAD) {
         animation_set(&self->anim, &anim_flyer_death);
@@ -289,6 +322,9 @@ static void charger_update(Entity *self, float dt, const Tilemap *map) {
 
     Body *body = &self->body;
 
+    /* Kill if fallen off bottom of level */
+    if (enemy_check_level_bottom(self, map, s_charger_em)) return;
+
     /* Death sequence */
     if (self->flags & ENTITY_DEAD) {
         animation_set(&self->anim, &anim_charger_death);
@@ -319,14 +355,10 @@ static void charger_update(Entity *self, float dt, const Tilemap *map) {
             }
 
             /* Reverse at ledge */
-            if (body->on_ground) {
-                float cx = (cd->patrol_dir > 0) ?
-                    body->pos.x + body->size.x + 2.0f :
-                    body->pos.x - 2.0f;
-                float cy = body->pos.y + body->size.y + 4.0f;
-                if (!tilemap_is_solid(map, world_to_tile(cx), world_to_tile(cy))) {
-                    cd->patrol_dir = -cd->patrol_dir;
-                }
+            if (enemy_detect_cliff(body, cd->patrol_dir,
+                                    CHARGER_PATROL_SPEED, map)) {
+                cd->patrol_dir = -cd->patrol_dir;
+                body->vel.x = 0;
             }
 
             /* Detect player — horizontal line-of-sight */
@@ -510,10 +542,12 @@ static int count_alive_grunts(EntityManager *em) {
 }
 
 static void spawner_update(Entity *self, float dt, const Tilemap *map) {
-    (void)map;
     SpawnerData *sd = (SpawnerData *)self->data;
     if (!sd) return;
 
+    /* Kill if fallen off bottom of level */
+    if (enemy_check_level_bottom(self, map, s_spawner_em)) return;
+
     /* Death sequence */
     if (self->flags & ENTITY_DEAD) {
         animation_set(&self->anim, &anim_spawner_death);