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/perf/PerfOverlaySparkline.tsx

@@ -62,7 +62,7 @@ function scale(samples: readonly PerfLiveSample[], kind: SparklineKind): { min:
   const minVal = Math.min(...values);
   const maxVal = Math.max(...values);
   if (kind === 'cpu') {
-    return { min: 0, max: Math.max(100, maxVal * 1.1) };
+    return { min: 0, max: Math.max(5, maxVal * 1.25) };
   }
   const pad = Math.max((maxVal - minVal) * 0.08, maxVal * 0.02, 1);
   return { min: Math.max(0, minVal - pad), max: maxVal + pad };
@@ -74,8 +74,10 @@ function stableScale(
   peakRef: { current: number },
 ): { min: number; max: number } {
   const raw = scale(samples, kind);
-  if (kind === 'cpu') return raw;
   if (raw.max > peakRef.current) peakRef.current = raw.max;
+  if (kind === 'cpu') {
+    return { min: 0, max: Math.max(5, Math.max(peakRef.current, raw.max)) };
+  }
   return { min: 0, max: Math.max(peakRef.current, raw.max) };
 }