Add UGS Sessions online hosting with no port-forwarding

Players can now host and join over the internet without port-forwarding
via Unity Gaming Services Relay + Lobby, exposed through the unified
Multiplayer Services SDK (Sessions API).

- OnlineSessionManager: anonymous auth, CreateSessionAsync (public,
  relay-backed), QuerySessionsAsync server browser, JoinSessionByIdAsync,
  and session leave wired into the existing net-stop teardown.
- Per-install auth profile derived from the project path
  (mirror_<sha1(dataPath)>) so junction-mirror clones get distinct UGS
  player ids, avoiding the same-machine 409 LobbyConflict on join.
- Install com.unity.services.multiplayer 2.2.4 (pulls Relay/Lobby/
  Authentication/Core) and attach OnlineSessionManager to the
  NetworkManager in Main.

Loopback dev flow (NetworkBootstrap H/C/J/K) is left untouched; the
online path is additive. MatchManager and gameplay are unchanged since
Sessions starts the host/client through NGO as before.

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

Assets/Scenes/Main.unity

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