feat(perf): cover pipeline throughput (cpm) in performance probe (#945)

* feat(perf): cover pipeline throughput (cpm) in performance probe

Mirror the analysis pipeline's tpm for covers. A new coverPerfStore samples the
backfill `done` progress from cover:library-progress events and derives a rolling
one-minute covers-per-minute rate. Surfaced as a live diag in perfLiveStore, a
pinnable "Cover backfill" throughput card in the Monitor tab, and a cpm row in
the Cover pipeline overlay block.

* docs(changelog): note cover pipeline cpm metric (PR #945)

Add CHANGELOG entry and credits line for the cover-pipeline covers-per-minute
throughput metric in the Performance Probe.

src/utils/perf/coverPerfStore.test.ts

@@ -0,0 +1,51 @@
+import { describe, expect, it, beforeEach, vi, afterEach } from 'vitest';
+import {
+  getCoverCachedPerMinute,
+  getCoverPerfState,
+  recordCoverProgress,
+  resetCoverPerfStateForTest,
+} from './coverPerfStore';
+
+beforeEach(() => {
+  resetCoverPerfStateForTest();
+});
+
+afterEach(() => {
+  vi.useRealTimers();
+});
+
+describe('coverPerfStore', () => {
+  it('derives covers-per-minute from done deltas over time', () => {
+    vi.useFakeTimers();
+    vi.setSystemTime(1_000_000);
+    recordCoverProgress({ done: 100, total: 1000, pending: 900 });
+    vi.advanceTimersByTime(30_000);
+    recordCoverProgress({ done: 130, total: 1000, pending: 870 });
+    // +30 covers over 30s ≈ 60 cpm.
+    expect(getCoverCachedPerMinute()).toBeCloseTo(60, 0);
+    expect(getCoverPerfState().done).toBe(130);
+  });
+
+  it('returns 0 with a single sample and prunes the window after a minute', () => {
+    vi.useFakeTimers();
+    vi.setSystemTime(2_000_000);
+    recordCoverProgress({ done: 10 });
+    expect(getCoverCachedPerMinute()).toBe(0);
+    vi.advanceTimersByTime(20_000);
+    recordCoverProgress({ done: 20 });
+    expect(getCoverCachedPerMinute()).toBeGreaterThan(0);
+    vi.advanceTimersByTime(61_000);
+    expect(getCoverCachedPerMinute()).toBe(0);
+  });
+
+  it('resets the window on a backwards jump (server switch / cache clear)', () => {
+    vi.useFakeTimers();
+    vi.setSystemTime(3_000_000);
+    recordCoverProgress({ done: 500 });
+    vi.advanceTimersByTime(5_000);
+    recordCoverProgress({ done: 5 });
+    // Only the new baseline remains → no rate yet.
+    expect(getCoverCachedPerMinute()).toBe(0);
+    expect(getCoverPerfState().done).toBe(5);
+  });
+});