Introduce zero corner geometry helpers

[pascalpp]

Summary

• Add ZeroCornerGeometry as the shared helper layer for zero-corner traits and frame placement.
• Route align/reset placement and resize-handle math through the zero-corner model.
• Cover all four zero-corner states with focused core tests.

Validation

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

Closes #193.

[pascalpp]

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• Introduce zero corner geometry helpers #201 👈 (View in Graphite)
• Make rulers flippable #199 : 1 other dependent PR (#207 )
• main

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[Copilot]

Pull request overview

This PR introduces a centralized ZeroCornerGeometry helper to consistently derive zero-corner-dependent behavior (frame placement, growth direction, tick side, and resize-handle side) across ruler operations, and adds targeted unit tests to validate all four zero-corner states.

Changes:

• Add ZeroCornerGeometry (plus supporting RulerGrowthDirection/RulerSide) to model zero-corner traits and compute zero-point/frame placement.
• Route ruler alignment/reset positioning through ZeroCornerGeometry-backed frame placement.
• Update resize-handle placement and resize-frame math to respect the selected zero corner, with new unit tests covering additional resize directions.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
FreeRulerTests/RulerCoreTests.swift	Adds focused tests covering all four zero-corner states and new resize directions.
Free Ruler/RulerController.swift	Uses ZeroCornerGeometry for alignment and zero-corner-aware default positioning on reset.
Free Ruler/Ruler.swift	Introduces ZeroCornerGeometry and refactors default frame computation to use the helper.
Free Ruler/ResizeHandleView.swift	Makes resize-handle placement and resizing math depend on the zero-corner-derived resize side.

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