fix: lighten Resource History sampler, speed up loads, tidy shutdown

[laurentiu021]

What

Performance + robustness fixes for Resource History (#13), from the post-feature audit (Batch 3 of 6).

Fixes

1. Always-on sampler no longer does per-poll disk WMI + SMART (perf, HIGH). The 10s sampler called TemperatureService.ReadAllAsync(), which on an elevated machine runs a Win32_DiskDrive WMI query plus a full DiskHealthService SMART enumeration just to name storage sensors — data the sampler discards (it stores only CPU/GPU temps). Added ReadAllAsync(includeStorage: false) that skips that; the sampler uses it. ~8,640 needless WMI/SMART enumerations/day eliminated on a tray-minimized session. The Dashboard keeps the full read (default true) — no behavior change there.
2. LoadAsync parsed the whole file every time (perf, HIGH). Up to ~260k lines at 30-day retention, allocating a sample per line then discarding 99% for a "last hour" view. It now reads from the newest end and stops at the cutoff, bounding work to the requested window.
3. Dispose race (robustness). Dispose disposed _fileLock without awaiting the sampler loop, so an in-flight Release() could hit a disposed semaphore at shutdown (caught+logged, but a real race). Dispose now waits (bounded 2s) for the loop to cancel first, guards Release with _disposed, and PruneAsync takes a cancellation token.
4. Temperature EmptyState (UX). The temperature chart now shows a "no temperature data" message on sensorless machines instead of a blank chart.

Tests

Downsample boundary tests: maxPoints 0 (clamp), 1 (averages whole series), exactly-N (unchanged), partial-GPU-nulls (averages over present values only).

Docs

CHANGELOG 1.51.3, version bump to 1.51.3.

Part of the post-feature audit remediation (Batch 3 of 6).