perf(analysis): measure tpm over trailing 5s window (#948)

* perf(analysis): measure tpm over trailing 5s instead of full minute

Mirror the cover cpm change: a 60s rolling average added too much inertia and
flattened real bursts/stalls. Count completions in the trailing 5s window and
extrapolate to per-minute, so analysis tpm reacts promptly and decays to 0
within the window when idle. Retention stays at 60s.

* docs(changelog): note trailing-window throughput rate (PR #948)

src/utils/perf/analysisPerfStore.test.ts

@@ -14,31 +14,36 @@ afterEach(() => {
 });
 
 describe('analysisPerfStore', () => {
-  it('records last track timings and rolling tpm', () => {
+  const record = (trackId: string): void =>
+    recordAnalysisTrackPerf({ trackId, fetchMs: 1, seedMs: 1, bpmMs: 1, totalMs: 3 });
+
+  it('records last track timings and trailing-window tpm', () => {
     vi.useFakeTimers();
     vi.setSystemTime(1_000_000);
-    recordAnalysisTrackPerf({
-      trackId: 't1',
-      fetchMs: 1000,
-      seedMs: 2000,
-      bpmMs: 500,
-      totalMs: 3500,
-    });
+    record('t1');
     vi.advanceTimersByTime(100);
-    expect(getAnalysisTracksPerMinute()).toBeGreaterThan(0);
+    // 1 completion in the trailing 5s → 1 / 5s × 60 = 12 tpm.
+    expect(getAnalysisTracksPerMinute()).toBeCloseTo(12, 0);
   });
 
-  it('prunes completions older than one minute from tpm window', () => {
+  it('extrapolates a burst over the trailing 5s window', () => {
     vi.useFakeTimers();
     vi.setSystemTime(2_000_000);
-    recordAnalysisTrackPerf({
-      trackId: 'old',
-      fetchMs: 1,
-      seedMs: 1,
-      bpmMs: 1,
-      totalMs: 3,
-    });
-    vi.advanceTimersByTime(61_000);
+    record('a');
+    vi.advanceTimersByTime(1_000);
+    record('b');
+    vi.advanceTimersByTime(1_000);
+    record('c');
+    // 3 completions within the last 5s → 3 / 5s × 60 = 36 tpm.
+    expect(getAnalysisTracksPerMinute()).toBeCloseTo(36, 0);
+  });
+
+  it('decays to 0 once completions fall outside the trailing window', () => {
+    vi.useFakeTimers();
+    vi.setSystemTime(3_000_000);
+    record('old');
+    expect(getAnalysisTracksPerMinute()).toBeGreaterThan(0);
+    vi.advanceTimersByTime(6_000);
     expect(getAnalysisTracksPerMinute()).toBe(0);
   });
 });