Verify flippable ruler zero corner integration

[pascalpp]

Summary

Perform final integration QA for the flippable ruler zero-corner feature.

Verification Checklist

• Verify all four zero-corner states visually.
• Verify grouped and ungrouped Flip Horizontal / Flip Vertical behavior.
• Verify resize handles and resize math in each state.
• Verify mouse tick labels in pixels, millimeters, and inches.
• Verify Align Rulers at Mouse Location aligns the active shared zero corner.
• Verify Reset Ruler Positions preserves flip state and lays out a coherent corner.
• Verify app relaunch persistence.
• Verify hidden/showing ruler behavior.

Required Test Command

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

Acceptance Criteria

• Core tests pass.
• Manual QA checklist is complete.
• Any follow-up bugs found during integration are filed or fixed before closing the parent issue.

[pascalpp]

Closed by #206