Fix SMAppService helper so Performance Mode can control fans

[nsd97]

What does this PR do?

Makes Performance Mode actually drive fans by fixing SMAppService daemon packaging (BundleProgram + LaunchDaemons embed), gating UI/control on a live helper, and only caching fan targets after successful XPC — closing the “UI lies while fans stay on auto” bug.

Closes #4.

Context

• Issue: Performance Mode shows controlled fans when helper is not running #4
• Stack: Part 2/4 — depends on Add Swift Testing harness and SwiftUI previews #5 (Swift Testing harness). This branch includes that commit until Add Swift Testing harness and SwiftUI previews #5 merges; please review/merge Add Swift Testing harness and SwiftUI previews #5 first, then this PR shrinks to the helper fix.
• Out of scope: Ultra performance level, relocating the helper binary to Contents/Resources, SwiftUI popover monitor deferral (separate PR)

What changed

• FanControlHelper/Launchd.plist: add BundleProgram → Contents/Library/LaunchServices/com.idevtim.ChillMac.Helper
• project.yml: copy plist into Contents/Library/LaunchDaemons/com.idevtim.ChillMac.Helper.plist; declare helper product as copy-script input (sandbox)
• Gate helperReady / Performance Mode on live helper readiness; Install Helper CTA + Login Items deep link when down
• FanControlPolicy + extract PerformanceCurve (Low→Max only); only commit target caches after successful helper reply; skip no-op @Published override writes
• Unit tests for policy, launchd plist layout, and Max curve regression

Docs vs forums (why readiness ≠ .enabled alone)

Apple documents SMAppService.Status.enabled as registered/eligible. Forum/DTS experience: .enabled is necessary but not sufficient — the daemon can be absent and XPC can fail. This PR gates on usable helper behavior and surfaces Install / Login Items when it is not.

Review guide

Start at: FanControlHelper/Launchd.plist → project.yml (LaunchDaemons copy) → ChillMac/Fan/FanControlPolicy.swift → ChillMac/App/main.swift / PopoverView Install CTA → HelperConnection error completions

How to test

1. xcodegen generate && xcodebuild -project ChillMac.xcodeproj -scheme ChillMac -destination 'platform=macOS' test
2. Before fix (or with helper revoked): System Settings → Login Items → turn ChillMac background item off → open popover → Performance Mode should not pretend to control fans; Install CTA visible; fan RPM stays macOS-auto
3. After fix: tap Install Helper → approve in Login Items if prompted → enable Performance Mode → fans follow the curve (RPM moves with level/temp)
4. Confirm helper binary still lives at Contents/Library/LaunchServices/ (intentional SMJobBless-era path; not moved to Contents/Resources)

Checklist

• Builds without errors (xcodegen generate && xcodebuild -scheme ChillMac build)
• Unit tests pass (xcodebuild -project ChillMac.xcodeproj -scheme ChillMac -destination 'platform=macOS' test)
• Tested manually in the app

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