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/components/FpsOverlay.tsx

@@ -32,7 +32,7 @@ import {
 } from '../utils/perf/perfOverlayMode';
 import { useAnalysisPerfListener } from '../hooks/useAnalysisPerfListener';
 import { useCoverPerfListener } from '../hooks/useCoverPerfListener';
-import { getCoverCachedPerMinute } from '../utils/perf/coverPerfStore';
+import { getCoverCachedPerMinute, getCoverUiPerMinute } from '../utils/perf/coverPerfStore';
 
 const SAMPLE_MS = 500;
 const TPM_REFRESH_MS = 500;
@@ -69,6 +69,7 @@ export default function FpsOverlay() {
   const [fps, setFps] = useState(0);
   const [tpm, setTpm] = useState(0);
   const [cpm, setCpm] = useState(0);
+  const [cpmUi, setCpmUi] = useState(0);
   const [queueStats, setQueueStats] = useState<AnalysisPipelineQueueStatsDto | null>(null);
   const [coverQueueLines, setCoverQueueLines] = useState<string[]>([]);
   const last = useAnalysisPerfLast();
@@ -135,9 +136,13 @@ export default function FpsOverlay() {
   useEffect(() => {
     if (!showCoverPerfOverlay) {
       setCpm(0);
+      setCpmUi(0);
       return;
     }
-    const refresh = () => setCpm(getCoverCachedPerMinute());
+    const refresh = () => {
+      setCpm(getCoverCachedPerMinute());
+      setCpmUi(getCoverUiPerMinute());
+    };
     refresh();
     const id = window.setInterval(refresh, TPM_REFRESH_MS);
     return () => window.clearInterval(id);
@@ -270,7 +275,12 @@ export default function FpsOverlay() {
           <div className="fps-overlay__row">
             {cpm.toFixed(1)}
             {' '}
-            <span className="fps-overlay__unit">cpm</span>
+            <span className="fps-overlay__unit">lib cpm</span>
+          </div>
+          <div className="fps-overlay__row">
+            {cpmUi.toFixed(1)}
+            {' '}
+            <span className="fps-overlay__unit">ui cpm</span>
           </div>
           {coverQueueLines.length > 0 ? coverQueueLines.map(line => (
             <div key={line} className="fps-overlay__row fps-overlay__row--steps">

src/components/sidebar/perfProbe/SidebarPerfProbeMonitorTab.tsx

@@ -223,9 +223,9 @@ export default function SidebarPerfProbeMonitorTab() {
       )}
 
       {live.cover && (
-        <PerfProbeMetricSection title="Cover backfill" defaultOpen={false}>
+        <PerfProbeMetricSection title="Cover pipeline" defaultOpen={false}>
           <PerfProbeMetricCard
-            label="Throughput"
+            label="Backfill (lib)"
             value={live.cover.cachedPerMinute.toFixed(1)}
             unit="cpm"
             detail={live.cover.total > 0
@@ -234,6 +234,14 @@ export default function SidebarPerfProbeMonitorTab() {
             pinned={livePinned('cover:cpm')}
             onTogglePin={toggleLive('cover:cpm')}
           />
+          <PerfProbeMetricCard
+            label="On-demand (ui)"
+            value={live.cover.uiPerMinute.toFixed(1)}
+            unit="cpm"
+            detail="UI cover ensures per minute"
+            pinned={livePinned('cover:cpm:ui')}
+            onTogglePin={toggleLive('cover:cpm:ui')}
+          />
         </PerfProbeMetricSection>
       )}
     </div>