fix(perf): reduce idle Rust CPU and stabilize Performance Probe overlay (#939)

* fix(perf): skip Performance Probe CPU snapshot poll on Windows

Windows has no Rust CPU/RSS sampler, but the probe still invoked
performance_cpu_snapshot every 2s when the modal or overlay pins were
active. Skip the IPC on unsupported platforms and only poll JS-side
metrics; do not start overlay polling for CPU/memory pins alone.

* fix(analysis): park backfill coordinator until Advanced is configured

#881 started run_coordinator_forever at app init with a 2s sleep even when
disabled, waking tokio on every platform for no work. Park on Notify instead;
wake on configure (enable/disable) and library sync-idle. Long sleeps use
select with wake so sync-idle can interrupt COMPLETED_RECHECK waits.

Investigation branch — not for merge until periodic CPU root cause is confirmed.

* fix(perf): stop probe overlay flicker on live poll updates

Publish CPU samples only after a valid jiffies baseline, skip no-op
snapshot emits, and sync sparkline history atomically in the store.
Overlay uses wall-clock sparkline time and auto-scales low CPU values.

* fix(perf): cut idle Rust CPU from probe scan, cover prefetch, and storage poll

Move performance_cpu_snapshot /proc work to spawn_blocking so tokio
workers are not charged with probe sampling. Stop lazy cover strategy
from running route prefetch disk stats every 1.5s, and slow hot-cache
size refresh on Settings → Storage to 15s.

* fix(perf): stabilize probe sparkline clock between live poll ticks

Track sampleAt separately from updatedAt so CPU rate history and overlay
sparklines only advance on real % changes, not FPS re-renders or RSS-only
poll ticks. Hold CPU sparkline Y scale with a peak ref to avoid scale jumps.

* fix(cover): restore lazy route prefetch without idle disk stats poll

Re-enable lazy cover registry warm-up so cached WebP paths reach
diskSrcCache before cells mount. Skip cover_cache_stats on every 1.5s
tick — drain batches via ensure only, poll full disk usage every 30s
when the registry is idle.

* docs: CHANGELOG and credits for PR #939 idle CPU perf fix

* fix(cover): peek before route prefetch ensure to match main responsiveness

Route prefetch moved batch drain ahead of cover_cache_stats for idle CPU,
which removed the accidental throttle and flooded ensure invoke slots.
Use warmCoverDiskSrcBatch first (cached hits skip ensure), ensure misses
only, and yield while high-priority viewport work is queued.

src/components/sidebar/perfProbe/SidebarPerfProbeMonitorTab.tsx

@@ -1,5 +1,5 @@
 import { useMemo, useRef } from 'react';
-import { usePerfLiveSnapshot } from '../../../utils/perf/perfLiveStore';
+import { isPerfLivePollWaitingForCpu, usePerfLiveSnapshot } from '../../../utils/perf/perfLiveStore';
 import { usePerfLiveIncludeThreadGroups } from '../../../utils/perf/perfLivePollSettings';
 import {
   togglePerfLiveOverlayPin,
@@ -26,7 +26,7 @@ export default function SidebarPerfProbeMonitorTab() {
   const coverPinned = usePipelineOverlayPinned('pipeline:cover');
   const cpu = live.cpu;
   const cpuSupported = cpu?.supported === true;
-  const collecting = live.collecting && cpu == null;
+  const collecting = isPerfLivePollWaitingForCpu();
   const includeThreadGroups = usePerfLiveIncludeThreadGroups();
   const peakMemoryKbRef = useRef(1);
   const peakThreadCpuRef = useRef(1);