Seal boss stairs after victory

docs/bosses.md

@@ -41,6 +41,7 @@ Killing a boss grants **500 + floor × 100** score, clears all hostile bullets/t
 - **1 item pedestal** (left of room center)
 - **1 weapon drop** rolled at **floor + 2** (right of center)
 - **1 ammo pickup** (above center) — the boss room also stocks one when you walk in, so you can afford to spend the magazine on the fight
+- **1 sealed stair hatch** (below center) — it remains closed and uncollectable for 3 seconds after the kill so the player cannot fall through it accidentally
 - **Stairs Down** (below center)
 
 > *"[NAME] DESTROYED — take the meat, take the stairs"*