Fix downward dash not damaging enemies and add post-dash invincibility

Stomping was guarded by the invincibility check, so during a downward
dash the player could never deal stomp damage. Move the invincibility
guard to only protect against taking damage, not dealing it.

Extend dash invincibility by PLAYER_DASH_INV_GRACE (0.15s) past the
dash duration so the player is briefly protected after landing.

Closes #15

src/game/level.c

@@ -306,8 +306,7 @@ static void handle_collisions(EntityManager *em) {
         }
 
         /* ── Enemy contact damage to player ──── */
-        if (player && !(player->flags & ENTITY_INVINCIBLE) &&
-            entity_is_enemy(a) && !(a->flags & ENTITY_DEAD)) {
+        if (player && entity_is_enemy(a) && !(a->flags & ENTITY_DEAD)) {
             if (physics_overlap(&a->body, &player->body)) {
                 /* Check if player is stomping (falling onto enemy from above) */
                 bool stomping = (player->body.vel.y > 0) &&
@@ -319,7 +318,7 @@ static void handle_collisions(EntityManager *em) {
                     damage_entity(a, 2);
                     if (a->flags & ENTITY_DEAD) stats_record_kill();
                     player->body.vel.y = -PLAYER_JUMP_FORCE * 0.7f;
-                } else {
+                } else if (!(player->flags & ENTITY_INVINCIBLE)) {
                     /* Charger deals extra damage and knockback while charging */
                     int dmg = a->damage;
                     if (a->type == ENT_ENEMY_CHARGER) {