fix: tiny anti-instant-death grace on block timeout (random timing preserved)

Random block timing is the core mechanic and is intentionally NOT softened —
early entry is a knowingly-risky bet. The only change is a small configurable
grace (GAME_START_GRACE_MS, default 2000ms, 0 to disable) so a block landing
in the same instant a game starts can't kill the player before the dungeon
even renders or before their first move is possible (100ms move cooldown).

(Replaces an earlier over-reaching "uniform time budget" idea that would have
defeated the random-timing mechanic.)

src/.env.example

@@ -348,6 +348,12 @@ RPC_POLL_INTERVAL=2000
 # req.ip. Leave unset/false for direct exposure, otherwise clients could spoof their IP.
 # TRUST_PROXY=true
 
+# Anti-instant-death grace (ms) before a block-timeout can end a game. This is NOT a
+# fairness floor — random block timing is the core mechanic. It only prevents the
+# degenerate case where a block lands the instant a game starts (before the dungeon
+# renders / before the first move is possible). Default 2000 (2s); set 0 to disable.
+# GAME_START_GRACE_MS=2000
+
 # =============================================================================
 # SECTION 12: SERVER & MULTI-INSTANCE
 # =============================================================================