Add persistent per-install player identity

Introduce PersistentPlayerId: a stable id that survives an app restart and is
unique per install (built on the UGS install-path-namespaced anonymous
PlayerId, with a local PlayerPrefs GUID fallback for loopback). Clones on the
same machine stay distinct.

Each client reports its id to the server on connect via a new Match_Identity
named message (mirroring Match_ReportHit, so no ConnectionApproval timing
risk); the host records its own directly. The server keeps an ephemeral
clientId -> persistent id map, exposed via PersistentIdOf(clientId) as the
foundation for later reconnect recognition. Map entries are cleared on
disconnect and net stop.

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

Assets/Scripts/Network/PersistentPlayerId.cs.meta

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