Persist zero-corner preference

[pascalpp]

Summary

• Add the four-state ZeroCorner model for ruler zero-corner selection
• Persist prefs.zeroCorner through UserDefaults, defaulting invalid or missing values to top-left
• Reset zero-corner state during UI-test defaults reset
• Add core tests for raw-value stability, fallback loading, and preference write-through

Validation

• xcodebuild -project "Free Ruler.xcodeproj" -scheme "Free Ruler" test -only-testing:FreeRulerTests

Stacked on #199 for the first child issue from #192: Persist zero-corner state.

[Copilot]

Pull request overview

This PR introduces a persisted “zero corner” preference (four-state model) so the app can remember which corner acts as the shared ruler origin across launches, and ensures UI test runs reset that preference deterministically.

Changes:

• Add a four-state ZeroCorner model and expose it as a new Prefs.zeroCorner preference.
• Persist prefs.zeroCorner via UserDefaults with fallback-to-.topLeft loading.
• Add unit tests validating raw-value stability, fallback loading, and write-through to UserDefaults.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
FreeRulerTests/RulerCoreTests.swift	Adds tests for ZeroCorner raw values, fallback loading, and persistence to UserDefaults (plus a restore helper).
Free Ruler/Ruler.swift	Introduces the ZeroCorner enum used by preferences and persistence.
Free Ruler/Prefs.swift	Adds zeroCorner to default registration, loads it with fallback, and persists changes via KVO observation.
Free Ruler/AppDelegate.swift	Resets zeroCorner during UI-test state reset alongside other preferences.

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