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.

1 files changed, 5 insertions, 0 deletions

diff --git a/src/features/plugins/ui/WorkspacePluginModal.svelte b/src/features/plugins/ui/WorkspacePluginModal.svelte
index d1bb27a..0ba7cd6 100644
--- a/src/features/plugins/ui/WorkspacePluginModal.svelte
+++ b/src/features/plugins/ui/WorkspacePluginModal.svelte
@@ -38,6 +38,11 @@
 
   async function toggle(pluginId: string, checked: boolean): Promise<void> {
     await plugins.setWorkspaceState(workspaceId, pluginId, checked ? "enabled" : "disabled");
+    // The per-workspace activation rule (layout): toggling a plugin may have
+    // changed which layout plugin serves THIS workspace — re-probe so the
+    // newly-enabled provider's tree (or the no-provider degrade) renders
+    // without a reload. The store ignores non-active workspaces.
+    layout?.refresh(workspaceId);
   }
 
   // ── The reset affordance (plan 04 §4.4) ─────────────────────────────────