feat(playground): knight armor pass + worker stale window bumped to 12s

Worker (deployed):
- STALE_AFTER_MS 5000 -> 12000 to forgive bg-tab throttling.

Player avatar:
- Muscled cuirass chest plate + glowing winged Hermes sigil disc on the chest
- Tasset (4 armored skirt strips) at the pelvis
- Steel gauntlets on the forearms
- Greaves (shin armor) on each leg
- Shoulder pauldron stud kept; helmet/cape/weapon variants unchanged

Result: avatars now read as actual knights, not blobs.

playground-ws-worker/src/worker.ts

@@ -35,7 +35,7 @@ interface PresenceMsg {
   [key: string]: unknown
 }
 
-const STALE_AFTER_MS = 5000
+const STALE_AFTER_MS = 12000 // bumped from 5000 — forgive bg-tab throttling so avatars don't flicker
 const CHAT_RING_MAX = 50
 const PRESENCE_DEDUPE_MS = 50
 const RATE_BUCKET_CAP = 30 // msgs