Diff-state consumer (part 2): RemoteDiffStateModel backend + Remote enum variant

[jwp2987]

Part 2 of the code-review diff-state consumer split (oracle-style). Adds the SSH-remote backend the DiffStateModel enum dispatches to for remote repositories, adapted to the fork's per-host-client transport.

What changed

RemoteDiffStateModel (app/src/code_review/diff_state_remote.rs)

• Construction issues an initial GetDiffState subscription RPC to the host's daemon (RemoteServerManager::client_for_host) and stores the returned snapshot; subscribes to the manager push channel so later DiffState{Snapshot,MetadataUpdate,FileDelta}Received events fold into the stored state (filtered by host + repo_path + mode). Reconnect (HostConnected) re-subscribes.
• Presents the same read API as LocalDiffStateModel (mirrors its metadata/mode accessors exactly) and emits the same DiffStateModelEvents, so the enum wrapper and the code-review view are backend-agnostic.
• Mutations: set_diff_mode unsubscribes the old (repo, mode) and subscribes the new; load_diffs re-requests; discard/refresh are daemon-owned no-ops for now (the DiscardFiles RPC is a later increment); is_git_operation_blocked is always false.

Enum wiring (diff_state.rs)

• DiffStateModel gains Remote(ModelHandle<RemoteDiffStateModel>) + new_remote, and every forwarder gains a Self::Remote arm. The Remote parts are #[cfg(not(target_family = "wasm"))] (native-only — the manager is native), so wasm keeps the Local-only shape.

Decode API (diff_state_proto.rs)

• pub(crate) decode_snapshot / decode_metadata_update / decode_file_delta / proto_to_git_diff_with_base_content / encode_diff_mode for the consumer (it lives in code_review and can't use the pub(super) converters). Base content (proto content_at_base) is preserved onto FileDiffAndContent::content_at_head.

The Remote backend is not yet constructed anywhere — the remote-repo pane wiring (working_directories keying by host / LocalOrRemotePath) is increment 5c, which carries a design decision and is left for the maintainer.

Verification

• warp lib compiles (gui,tui,local_fs,test-util)
• remote_server::diff_state_proto 25/0 (4 new decode tests)
• code_review 76/0

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