feat(pane_group): key diff_state_models by LocalOrRemotePath + remote dispatch (5c slice)

[jwp2987]

First slice of the diff-state pane wiring (5c). Migrates the global diff_state_models map from PathBuf to LocalOrRemotePath and makes get_or_create_diff_state_model local/remote-aware, matching Warp:

• Local key → local-backed DiffStateModel::new.
• Remote key → bridge the warp_util RemotePath to the buffer_location family (util_remote_path_to_buffer), require a connected session for the host (RemoteServerManager::client_for_host, else None so the panel reads unavailable), then DiffStateModel::new_remote.

drop_unused_diff_state_models still takes local PathBuf roots and drops the matching Local keys; the 5 call sites wrap their local repo path in LocalOrRemotePath::Local.

Behavior-preserving — only Local keys are produced today, so this is functionally identical; it makes new_remote reachable through working_directories for when remote repo-root detection lands.

Remaining for 5c (documented, task zerx-lab#136)

The per-pane-group root maps (repository_roots / directory_to_terminal / pane_groups / focused_repo / code_review_views) + the WorkingDirectoriesEvent payloads are still PathBuf; and the real blocker — the fork has no remote repo-root detection at the pane layer (the detection pipeline resolves only local roots via DetectedRepositories), so producing Remote keys needs a new remote-detection path. That carries a design decision and is left for the maintainer.

Verification

• warp lib compiles (gui,tui,local_fs,test-util)
• working_directories 2/0, code_review 76/0

🤖 Generated with Claude Code