Diff-state over SSH: live push on repository change

[jwp2987]

Completes the daemon side of code-review diff-state over SSH. After the initial subscribe snapshot (PR #61), the daemon now pushes fresh snapshots as the repository changes, so a remote code-review view stays live.

What changed

• ServerModel gains a per-connection subscription registry (diff_state_subscriptions: conn -> Vec<DiffStateSubscription>). Each subscription stores the canonicalized repo path (matched against repository-change events) and the exact wire path string the client sent (echoed in pushed snapshots so the client's RemoteDiffStateModel keys on it).
• handle_get_diff_state registers the subscription after computing the initial snapshot; handle_unsubscribe_diff_state removes it; deregister_connection drops all of a connection's subscriptions on teardown.
• On RepoMetadataEvent::RepositoryUpdated (the daemon already subscribes to it), push_diff_state_for_repo recomputes and pushes a DiffStateSnapshot (no request_id — an unsolicited push, handled by the client's push_message_to_event) to every connection subscribed to that repo.
• The subscribe reply and the live push share snapshot_from_parts so the Loaded/Error/NotInRepository decision is identical.

MetadataUpdate / FileDelta (finer-grained, debounced) pushes remain available on the wire for a future optimization; RepositoryUpdated-driven full snapshots are the correct, debounce-free baseline.

Verification

• warp lib compiles (gui,tui,local_fs,test-util)
• remote_server 46/0 (incl. new snapshot_from_parts test)
• code_review 76/0

Note

This makes the manager DiffState*Received push events (wired in PR #61) live end-to-end once the remote consumer is constructed. The remaining piece — wiring remote repos into the code-review pane (increment 5c) — carries a design decision and is left for the maintainer.

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