Increment 1: persistent metric history + extended time-range charts

- MetricsStore: thread-safe on-disk ring buffer (Application Support), throttled
  background saves, flush on quit/background, 24h/20k cap
- MetricsMath: pure trim + downsample (unit-tested)
- HistoryRange (Live/5m/30m/1h/24h) + HistoryRangePicker + MetricTrendChart
- Wire recording into SystemMonitor; range picker in CPU/Memory/Network tabs
- Fix durability: flush history on applicationWillTerminate + scenePhase.background
- 8 new tests (30 total green); runtime smoke: no crash, 7 samples persisted

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

TopManager.xcodeproj/project.pbxproj

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