Add match-start readiness barrier (Loading phase)

Gate the Lobby-to-world transition on a new MatchSubPhase.Loading before
Hide: movement stays frozen and the Hide countdown does not start until
every connected client has spawned all match avatars and applied their
paintings.

- MatchManager: Loading phase; matchLoadTimeout tunable; Match_BeginLoad
  broadcasts the respawned avatar NetworkObjectIds (+ generation tag),
  Match_Ready collects per-client ACKs (host self-checks locally); barrier
  coroutine in RunMatchPhases before Hide; disconnect/teardown cleanup.
- GameModeManager: freeze local player (movement/paint/tab) during Loading,
  restored via ActiveMode.Enter() on transition to Hide (no camera jump).
- NetworkPlayerSetup: treat Loading like Hide in ComputeVisible so a
  fast-loading client cannot reveal a hider early.
- MatchHud: "Loading" phase label.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Assets/Scripts/Network/UI/MatchHud.cs

@@ -104,6 +104,7 @@ public class MatchHud : MonoBehaviour
     {
         switch (phase)
         {
+            case MatchManager.MatchSubPhase.Loading: return "Loading";
             case MatchManager.MatchSubPhase.Hide: return "Hide";
             case MatchManager.MatchSubPhase.Seek: return "Seek";
             case MatchManager.MatchSubPhase.Resolution: return "Resolution";