Add standalone workspaces

[nedtwigg]

Dormouse Standalone needs to have a concept of "workspaces". For now they will live across the top app bar by default, probably an option to make them a vertical sidebar list someday too.

Dormouse VS Code already has this feature - when you do Dormouse: Focus that opens the default workspace which is always open. When you do Dormouse: Open in Editor that opens a new Dormouse tab which is modeled as a new workspace.

[dormouse-bot]

This is stages 3 and 4 of the already-specced workspaces-rollout scope — docs/specs/layout.md ## Future. Stage 3 is exactly the top-of-app-bar strip described here (one tab per Workspace, union bell/TODO on inactive tabs only), and standalone.md already names AppBar.tsx as its landing site. Stage 4 is the real switching.

The model underneath is built and dormant behind dormouse.flags.workspaces (off by default): createWorkspace / closeWorkspace / renameWorkspace / setActiveWorkspace in workspace-store.ts, the union projection in workspace-union.ts, and PersistedWorkspace / PersistedWindow in session-types.ts. setActiveWorkspace flips the active id but nothing re-renders the Wall — Wall.tsx has no workspace wiring at all yet, so stage 4 is the bulk of the work. The primitive it needs does exist: mountElement / unmountElement detach a terminal's element while keeping the registry entry and PTY alive, which is what lets an inactive Workspace keep running (and keep ringing).

Two things worth deciding before the strip goes in:

Standalone persists no session state today. TauriAdapter.PERSIST_SESSION = false — every launch starts fresh by deliberate design, with the PersistedWindow plumbing built but gated off at the adapter boundary. So workspaces would be per-launch until that flag flips, which is probably not what "workspaces" implies to a user coming from the VS Code side, where each webview does persist. Whether stage 3 ships with restoration or without it is a real fork.

The VS Code parallel is shallower than it looks. VS Code deliberately does not use PersistedWindow — each webview persists exactly one bare PersistedSession, and VS Code (not Dormouse) owns tab creation, closing, and ordering, so there are no create/rename/close affordances there to copy. Standalone is the first host that needs a Window-level container plus its own lifecycle UX, so the strip is new surface rather than a port.

Both stages have an open "settled in the Storybook UI pass" gate on tab visuals and the command-mode switch/create/close/rename bindings (shortcuts.md reserves a slot for them, following the tmux window bindings), so I've left this as a tracker rather than opening a PR against an undecided design.