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/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);