feat(fm): remote View/Edit — native editor (daemon) + working-copy save-back (classic SSH)

[iret77]

What

Completes remote View/Edit (F3/F4) in the MC-style file manager for both remote host types. Local View/Edit shipped in #32; this PR adds the two remote paths:

Host type	How editing works	Save-back
Daemon host (⚡ resilient / devhost)	Opened natively in the code editor, buffer-synced with the SSH daemon	Real, version-checked save-back via the daemon (same path as the sidebar server browser)
Classic SSH-only host (no daemon)	Downloaded to a local working copy, opened in the native editor	SFTP upload back on every save

This closes the last mandatory gap in the file-manager Pflichtliste.

Host type 1 — daemon host (native)

The file manager is keyed by SSH node_id; the native editor needs a daemon HostId, which is opaque and only known after the daemon handshake. Bridged pull-based, observe-only:

• Record node_id → daemon SessionId on connect (try_open_daemon_ssh_terminal) / adopt (adopt_daemon_session).
• Resolve a live HostId on demand (daemon_host_for_node) via RemoteServerManager::{host_id_for_session, client_for_host}.

Self-correcting (session gone → graceful fallback) and never wrong (SessionIds are monotonic, never reused). The daemon session/connect flow is untouched — zero blast radius on the persistence feature.

Host type 2 — classic SSH (working copy + save-back)

Additive, low blast radius — one new event subscription, no editor surgery:

• FileManagerRegistry::backend_for_namespace lends the workspace a live SFTP connection already open for the host (no second connect).
• begin_remote_sftp_edit downloads the file into a unique local working copy (original filename kept for language + tab title), opens it via add_tab_for_code_file, and records it in Workspace::remote_sftp_edits keyed by the canonical local path — holding its own backend Arc so save-back survives the file-manager pane closing.
• A single subscription to GlobalBufferModelEvent::FileSaved drives on_remote_working_copy_saved: if the saved path is a tracked working copy, upload it back. Ordinary local saves return immediately. A save landing mid-upload is coalesced (resave_pending) and re-run, so none is dropped.

No-data-loss contract

• The remote file is only ever written by a save-back upload, never on open.
• A failed upload keeps the working copy and shows a prominent, persistent error — the editor already flagged the local save as done, so a silent upload failure would be dangerous.
• Concurrent external edits are last-write-wins on the next save — inherent to editing over plain SFTP (daemon hosts get the version-checked native path instead). Documented in code.

Working copies live under the OS temp dir in unique per-edit dirs (pid + counter, never reused; small text files, OS-reclaimed). No buffer-closed event exists to hook for eager deletion; stale map entries are harmless (a closed buffer emits no further save event → nothing uploads).

Verification

• cargo check -p warp — clean, 0 errors/warnings. build.rs enables local_tty + local_fs on non-wasm, so both new #[cfg(all(unix, feature = "local_tty"))] paths are actually compiled (the shipping feature set). The PR gate (cargo check --bin zaplex) exercises the same cfgs.
• cargo test -p warp --lib sftp_manager::fm_registry — 6 passed (incl. new backend_for_namespace); the whole lib compiles clean in test cfg too.
• Runtime acceptance:
  • Daemon host (devhost): F3/F4 → native editor; edit + Save round-trips.
  • Classic SSH host: F3/F4 → working copy opens; edit + Save uploads back to the host.

Files

• app/src/workspace/view.rs — daemon_node_sessions + daemon_host_for_node (native path); remote_sftp_edits + RemoteSftpEdit + begin_remote_sftp_edit + on_remote_working_copy_saved + FileSaved subscription (classic-SSH path); OpenFileInEditor routes through both.
• app/src/sftp_manager/fm_registry.rs — backend_for_namespace (+ unit test).

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