Support patternContentUnits=objectBoundingBox for pattern fills

[rursache]

Summary

• Implement patternContentUnits="objectBoundingBox" support for pattern fills
• When this attribute is set, pattern coordinates are in [0,1] range relative to the filled element's bounding box
• The pattern frame is now scaled to the shape's bounding box at render time
• Also fixes a bug in firstGraphicsElement(with:) where the search would stop prematurely at the first container element, even if the target element was a later sibling

Test plan

• Added testDOMPatternObjectBoundingBox() verifying objectBoundingBox patterns preserve content units
• Added testDOMPatternUserSpaceOnUse() verifying default patterns use userSpaceOnUse
• Added firstGraphicsElementSearchesPastContainers() verifying element lookup doesn't stop at containers
• All 196 tests pass

Closes #52

[codecov]

Codecov Report

❌ Patch coverage is 77.96610% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.37%. Comparing base (6238058) to head (ea017f7).
⚠️ Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
...Sources/LayerTree/LayerTree.CommandGenerator.swift	28.57%	10 Missing ⚠️
...wiftDraw/Sources/LayerTree/LayerTree.Pattern.swift	50.00%	2 Missing ⚠️
DOM/Sources/DOM.Use.swift	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #120      +/-   ##
==========================================
+ Coverage   89.02%   89.37%   +0.35%     
==========================================
  Files         159      159              
  Lines       11959    15441    +3482     
==========================================
+ Hits        10646    13801    +3155     
- Misses       1313     1640     +327     

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