Keep fans when closed on AC power

[nsd97]

What does this PR do?

Adds Keep Fans When Closed on Power (default off): on AC, leave fans under app control through display sleep / lock / willSleep so lid-close clamshell does not force fans back to auto.

Closes #11.

Context

• Issue: Proposal: Keep fans when closed on AC power #11
• Stack: standalone (no dependency on other open PRs)
• Out of scope: keeping the Mac awake (pmset / private clamshell SPI), battery lid-close beyond existing Keep Fans on Screen Sleep, Ultra / helper changes

What changed

• DisplaySleepFanPolicy — keepClosedOnPower && onAC → keep; else fall back to keepOnScreenSleep
• PowerSource.isOnAC via IOPS
• FanMonitor uses the same policy for screen sleep, screen lock, and willSleep (lid close often posts willSleep after screensDidSleep; resetting there was undoing keep)
• Settings → Fan Control → Keep Fans When Closed on Power
• Diagnostics export includes the new setting

Review guide

Start at: ChillMac/Fan/DisplaySleepFanPolicy.swift → ChillMac/Fan/FanMonitor.swift (handleSleep / handleScreenSleep / handleScreenLocked) → ChillMac/Views/SettingsView.swift (Fan Control row)

How to test

1. xcodegen generate && xcodebuild -project ChillMac.xcodeproj -scheme ChillMac build
2. Plug in AC → enable Performance Mode → Settings → Fan Control → turn Keep Fans When Closed on Power on
3. Close lid in clamshell (external display / awake) → fans should stay under app control (not snap to auto)
4. On battery with the new toggle on (and Keep Fans on Screen Sleep off) → lid close should still reset
5. Both keep toggles off → sleep / lid close still resets (unchanged)

Checklist

• Builds without errors (xcodegen generate && xcodebuild -scheme ChillMac build)
• Unit tests pass — N/A on current main (no test target yet; policy is pure and covered on the fork harness)
• Tested manually in the app

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