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/hooks/useCoverPerfListener.ts

@@ -1,6 +1,10 @@
 import { useEffect } from 'react';
 import { listen } from '@tauri-apps/api/event';
-import { recordCoverProgress } from '../utils/perf/coverPerfStore';
+import { coverGetPipelineQueueStats } from '../api/coverCache';
+import { recordCoverProgress, recordCoverUiTotal } from '../utils/perf/coverPerfStore';
+
+/** How often to sample the backend's cumulative on-demand (UI) ensure count. */
+const UI_POLL_MS = 1000;
 
 type CoverLibraryProgressPayload = {
   serverIndexKey?: string;
@@ -37,3 +41,28 @@ export function useCoverPerfListener(active: boolean): void {
     };
   }, [active]);
 }
+
+/**
+ * Poll the backend's cumulative on-demand (UI) ensure total so the cover store
+ * can derive a per-minute rate. Mount once (always-mounted probe hook) to avoid
+ * duplicate polling.
+ */
+export function useCoverUiThroughputPoll(active: boolean): void {
+  useEffect(() => {
+    if (!active) return;
+    let cancelled = false;
+    const sample = (): void => {
+      void coverGetPipelineQueueStats()
+        .then(stats => {
+          if (!cancelled) recordCoverUiTotal(stats.uiEnsuredTotal);
+        })
+        .catch(() => {});
+    };
+    sample();
+    const timer = window.setInterval(sample, UI_POLL_MS);
+    return () => {
+      cancelled = true;
+      window.clearInterval(timer);
+    };
+  }, [active]);
+}

src/hooks/useSidebarPerfProbe.ts

@@ -3,7 +3,7 @@ import { acquirePerfLivePoll, patchPerfLiveAnalysis } from '../utils/perf/perfLi
 import { setPerfProbeTelemetryActive } from '../utils/perf/perfTelemetry';
 import { useAnalysisPerfLast } from '../utils/perf/analysisPerfStore';
 import { useAnalysisPerfListener } from './useAnalysisPerfListener';
-import { useCoverPerfListener } from './useCoverPerfListener';
+import { useCoverPerfListener, useCoverUiThroughputPoll } from './useCoverPerfListener';
 import {
   getPerfProbeFlags,
   subscribePerfProbeFlags,
@@ -39,6 +39,7 @@ function useNeedCoverTelemetry(perfProbeOpen: boolean, livePins: ReadonlySet<str
       perfProbeOpen
       || getPerfProbeFlags().showCoverPerfOverlay
       || livePins.has('cover:cpm')
+      || livePins.has('cover:cpm:ui')
     ),
     () => perfProbeOpen,
   );
@@ -54,6 +55,7 @@ export function useSidebarPerfProbe(): Result {
 
   useAnalysisPerfListener(needAnalysis);
   useCoverPerfListener(needCover);
+  useCoverUiThroughputPoll(needCover);
 
   useEffect(() => {
     setPerfProbeTelemetryActive(perfProbeOpen);