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, 38 insertions, 0 deletions

diff --git a/src/features/layout/store.test.ts b/src/features/layout/store.test.ts
index 73850ed..331af95 100644
--- a/src/features/layout/store.test.ts
+++ b/src/features/layout/store.test.ts
@@ -315,6 +315,44 @@ describe("layout store — edits PUT the whole tree + rollback (§2.6)", () => {
 });
 
 describe("layout store — probe triggers (§2.6)", () => {
+  it("a per-workspace plugin-state change on the ACTIVE workspace re-probes (the plugins-modal toggle)", async () => {
+    let absent = true;
+    const served = overlaySidePanels(DEFAULT_LAYOUT, ["extensions"]);
+    const http = fakeHttp(() =>
+      absent
+        ? { ok: false, notFound: true, error: "No layout plugin is enabled in this workspace" }
+        : { ok: true, tree: served },
+    );
+    const store = makeStore(http);
+    await store.probe("ws-a");
+    expect(store.providerState).toBe("absent");
+
+    absent = false; // a layout plugin got enabled in THIS workspace
+    store.onWorkspacePluginsChanged("ws-a");
+    await vi.advanceTimersByTimeAsync(0);
+    expect(store.providerState).toBe("present");
+    expect(store.tree).toEqual(served);
+  });
+
+  it("a per-workspace plugin-state change on ANOTHER workspace is ignored (its probe fetches on switch)", async () => {
+    let absent = true;
+    const served = overlaySidePanels(DEFAULT_LAYOUT, ["extensions"]);
+    const http = fakeHttp(() =>
+      absent
+        ? { ok: false, notFound: true, error: "No layout plugin is enabled in this workspace" }
+        : { ok: true, tree: served },
+    );
+    const store = makeStore(http);
+    await store.probe("ws-a");
+    expect(store.providerState).toBe("absent");
+
+    absent = false; // a DIFFERENT workspace's toggle — nothing re-renders here
+    store.onWorkspacePluginsChanged("ws-other");
+    await vi.advanceTimersByTimeAsync(0);
+    expect(http.getCalls).toEqual(["ws-a"]); // no second probe
+    expect(store.providerState).toBe("absent");
+  });
+
   it("extensions-changed re-probes the active workspace", async () => {
     let absent = true;
     const served = overlaySidePanels(DEFAULT_LAYOUT, ["extensions"]);