Stagger new rulers from occupied defaults

[pascalpp]

Summary

• Staggers newly created rulers when the default zero point is already occupied.
• Uses a half-ruler-thickness diagonal offset so consecutive rulers remain visible instead of stacking exactly on top of each other.
• Covers the stagger behavior with a RulerManager unit test.

Tests

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

Fixes #247

[pascalpp]

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• Drag visible rulers together when grouped #254
• Stagger new rulers from occupied defaults #253 👈 (View in Graphite)
• feature: Support Multiple Rulers #246 : 9 other dependent PRs (#238 , #255 , #256 and 6 others)
• main

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[pascalpp]

[codex reviewer] Reviewed the staggered default-position change against the feature branch. No blocking feedback from me; the implementation only staggers exact default zero-point collisions, and the new unit test covers the multi-ruler collision case.