Update docs: acoustic taunt/whistle is implemented

Taunt sound plays on every radar ping (sweep + self-ping) on all
clients as positional 3D audio; move it from open questions to done
in GAME_DESIGN.md and note it in the README match-loop bullet.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -27,7 +27,7 @@ while hunters try to find them. Pure online PvP with an extended traversal kit
   painting with late-join replay, role prefab variants (hunter/hider).
 - **Match loop** (server-authoritative): hide → seek → resolve → back to lobby.
   Two modes: **Classic** (found hiders spectate) and **Conversion** (found hiders
-  become hunters). Hunter FP shooting, radar sweeps + self-ping taunt (E),
+  become hunters). Hunter FP shooting, radar sweeps + self-ping taunt (E) with positional taunt sound,
   X-ray reveal (anti-cheese via concealment measurement), conversion statues,
   match HUD and result screen with win evaluation.
 - **Third-person character controller** — camera-relative WASD, jump, sprint,