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/player.c

@@ -308,8 +308,8 @@ void player_update(Entity *self, float dt, const Tilemap *map) {
             pd->dash_timer = PLAYER_DASH_DURATION;
             pd->dash_dir   = vec2(0.0f, 1.0f);
 
-            /* Brief invincibility during dash */
-            pd->inv_timer = PLAYER_DASH_DURATION;
+            /* Invincibility during dash + short grace period after */
+            pd->inv_timer = PLAYER_DASH_DURATION + PLAYER_DASH_INV_GRACE;
             self->flags |= ENTITY_INVINCIBLE;
 
             /* Jetpack burst downward */