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.

CHANGELOG.md

@@ -365,6 +365,17 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Fixed
 
+### Performance — idle Rust CPU, probe overlay, and cover prefetch
+
+**By [@cucadmuh](https://github.com/cucadmuh), PR [#939](https://github.com/Psychotoxical/psysonic/pull/939)**
+
+* **Advanced analytics coordinator:** park on `Notify` when disabled — no idle 2s poll loop; wake on configure or library sync-idle.
+* **Performance Probe:** run CPU snapshot on the blocking pool; skip `/proc` poll on Windows; fix overlay flicker and sparkline clock jumps; hold previous CPU % until the first rate sample (no 0% flash).
+* **Background polls:** Settings → Storage hot-cache poll 15s; cover registry full disk stats every 30s when idle instead of every 1.5s tick.
+* **Cover art:** restore lazy route prefetch; batch disk peek before ensure so cached WebP warms `diskSrcCache` without flooding invoke slots; yield when viewport ensures are queued.
+
+
+
 ### CI — npmDepsHash on app-v* tags
 
 **By [@cucadmuh](https://github.com/cucadmuh), PR [#927](https://github.com/Psychotoxical/psysonic/pull/927)**