test: split multi-assertion specs into single-behavior tests

One behavior per test across FE machine/store specs and backend endpoint
tests, so a failure names exactly what broke.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/brief/application/brief.store.spec.ts

@@ -54,7 +54,7 @@ function setup(adapter: Partial<BriefAdapter>): BriefStore {
 }
 
 describe('BriefStore action state (Idle | Busy | Failed)', () => {
-  it('is Busy synchronously once a transition starts, then Idle on success', async () => {
+  it('is Busy synchronously once a transition starts', async () => {
     const approved: BriefView = {
       ...view,
       brief: { ...brief, status: { tag: 'approved', approvedBy: 'u2', approvedAt: 't' } },
@@ -70,7 +70,23 @@ describe('BriefStore action state (Idle | Busy | Failed)', () => {
     const pending = store.approve();
     expect(store.busy()).toBe(true); // set synchronously, before any await resolves
 
-    await pending;
+    await pending; // settle before the test ends
+  });
+
+  it('settles to Idle on a successful transition', async () => {
+    const approved: BriefView = {
+      ...view,
+      brief: { ...brief, status: { tag: 'approved', approvedBy: 'u2', approvedAt: 't' } },
+    };
+    const store = setup({
+      load: (): Promise<Result<string, BriefView>> => Promise.resolve({ ok: true, value: view }),
+      save: (): Promise<Result<string, BriefView>> => Promise.resolve({ ok: true, value: view }),
+      approve: (): Promise<Result<string, BriefView>> =>
+        Promise.resolve({ ok: true, value: approved }),
+    });
+    await store.load();
+
+    await store.approve();
     expect(store.busy()).toBe(false);
     expect(store.lastError()).toBeNull();
   });