feat(layout): re-probe the layout when the workspace's plugin states change

The backend now activates layout providers PER WORKSPACE (the workspace's
per-workspace enablement state picks the ONE active layout plugin), so
toggling a layout plugin in the workspace plugins modal changes which tree
serves the workspace. The modal's toggle now refreshes the layout so the
newly-enabled provider's tree (or the no-provider degrade) lands without a
reload:

- WorkspaceLayoutPort gains refresh(workspaceId) — the modal calls it
  after every state write (an attempted write re-probes either way: the
  probe is the idempotent catch-up; a refused write changed nothing, so it
  re-syncs the same tree).
- App.svelte wires the port to the layout store's new
  onWorkspacePluginsChanged(workspaceId) — a guarded re-probe that only
  fires for the ACTIVE workspace (another workspace's toggle is picked up
  by its own probe on switch, exactly like the other §2.6 triggers).

The per-workspace tree resolution itself is backend-side; the marker
renderer, the probe triggers, and the WS  push filtering are
unchanged.

Verification: typecheck (svelte-check 0 errors) / test (1528) / check
/ build all green.

0 files changed, 0 insertions, 0 deletions