feat(perf): add on-demand (ui) throughput to cover pipeline cpm (#947)

* feat(perf): add on-demand (ui) throughput to cover pipeline cpm

Cover cpm previously measured only the native backfill (lib) via the
cover:library-progress done delta. Add a parallel UI series: every completed
on-demand Rust cover ensure (grid / now-playing) records a timestamp, surfaced
as a covers-per-minute rate. Both are exposed in the live cover diag, shown as
separate Backfill (lib) / On-demand (ui) cards in the Monitor tab (each pinnable
to the overlay) and as lib/ui rows in the Cover pipeline overlay block.

* docs(changelog): note cover on-demand (ui) throughput (PR #947)

Add CHANGELOG entry and credits line for the UI cover cpm metric.

* fix(perf): source on-demand (ui) cpm from backend produced-cover count

The JS ensure-queue counter never tracked: produced covers return hit:true
(only misses/errors are hit:false), and ensure-queue dedup/HMR made client
counting unreliable. Count on-demand covers natively in ensure_inner on the
produce path (non-bulk, past the cache-hit gate), expose a cumulative
uiEnsuredTotal in the pipeline stats, and derive the per-minute rate on the
frontend from polled deltas — mirroring the lib backfill series.

* perf(cover): measure cpm over trailing 5s instead of full minute

A 60s rolling average added too much inertia, flattening real bursts and
stalls in both the lib backfill and on-demand (ui) cover throughput. Compute
the rate from the trailing 5s of samples (still extrapolated to per-minute),
so the figure reacts promptly and decays to 0 within the window when idle.

src/utils/perf/coverPerfStore.test.ts

@@ -2,7 +2,9 @@ import { describe, expect, it, beforeEach, vi, afterEach } from 'vitest';
 import {
   getCoverCachedPerMinute,
   getCoverPerfState,
+  getCoverUiPerMinute,
   recordCoverProgress,
+  recordCoverUiTotal,
   resetCoverPerfStateForTest,
 } from './coverPerfStore';
 
@@ -15,26 +17,29 @@ afterEach(() => {
 });
 
 describe('coverPerfStore', () => {
-  it('derives covers-per-minute from done deltas over time', () => {
+  it('derives covers-per-minute from done deltas over the trailing window', () => {
     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);
+    vi.advanceTimersByTime(1_000);
+    recordCoverProgress({ done: 110, total: 1000, pending: 890 });
+    vi.advanceTimersByTime(1_000);
+    recordCoverProgress({ done: 120, total: 1000, pending: 880 });
+    // +20 covers over the last 2s ≈ 600 cpm (no minute-long inertia).
+    expect(getCoverCachedPerMinute()).toBeCloseTo(600, 0);
+    expect(getCoverPerfState().done).toBe(120);
   });
 
-  it('returns 0 with a single sample and prunes the window after a minute', () => {
+  it('returns 0 with a single sample and decays once the trailing window empties', () => {
     vi.useFakeTimers();
     vi.setSystemTime(2_000_000);
     recordCoverProgress({ done: 10 });
     expect(getCoverCachedPerMinute()).toBe(0);
-    vi.advanceTimersByTime(20_000);
+    vi.advanceTimersByTime(1_000);
     recordCoverProgress({ done: 20 });
     expect(getCoverCachedPerMinute()).toBeGreaterThan(0);
-    vi.advanceTimersByTime(61_000);
+    // No fresh samples for >5s → trailing window empties → back to 0.
+    vi.advanceTimersByTime(6_000);
     expect(getCoverCachedPerMinute()).toBe(0);
   });
 
@@ -48,4 +53,31 @@ describe('coverPerfStore', () => {
     expect(getCoverCachedPerMinute()).toBe(0);
     expect(getCoverPerfState().done).toBe(5);
   });
+
+  it('derives UI covers-per-minute from backend total deltas over the trailing window', () => {
+    vi.useFakeTimers();
+    vi.setSystemTime(4_000_000);
+    expect(getCoverUiPerMinute()).toBe(0);
+    recordCoverUiTotal(100);
+    // A single sample has no delta yet.
+    expect(getCoverUiPerMinute()).toBe(0);
+    vi.advanceTimersByTime(2_000);
+    recordCoverUiTotal(130);
+    // +30 produced over the last 2s ≈ 900 cpm; lib series stays untouched.
+    expect(getCoverUiPerMinute()).toBeCloseTo(900, 0);
+    expect(getCoverCachedPerMinute()).toBe(0);
+    // Idle poll keeps reporting the same total → delta 0 → rate decays to 0.
+    vi.advanceTimersByTime(6_000);
+    recordCoverUiTotal(130);
+    expect(getCoverUiPerMinute()).toBe(0);
+  });
+
+  it('resets the UI window on a backwards jump (process restart)', () => {
+    vi.useFakeTimers();
+    vi.setSystemTime(5_000_000);
+    recordCoverUiTotal(500);
+    vi.advanceTimersByTime(5_000);
+    recordCoverUiTotal(3);
+    expect(getCoverUiPerMinute()).toBe(0);
+  });
 });