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"*

docs/gameplay.md

@@ -202,7 +202,7 @@ Clearing a combat room drops:
 |---|---|
 | Normal enemy | XP gems (enemy's XP value) · 5.25% + luck×3% ammo · 2.2% + luck×3% heart |
 | Elite enemy | 3× XP + 3–5 bonus gems · 18% + luck×10% **item** · else 45% + luck×15% ammo |
-| Boss | 1 item pedestal + 1 weapon (rolled at floor + 2) + 1 guaranteed ammo + stairs · the boss room also stocks 1 ammo when you enter |
+| Boss | 1 item pedestal + 1 weapon (rolled at floor + 2) + 1 guaranteed ammo + stairs · the boss room also stocks 1 ammo when you enter · the stair hatch stays sealed for 3 seconds after the kill |
 
 **Luck** is a player stat (Lucky Coin, Crow Bait, Scavenger perk) that scales most drop chances as shown above.