Support clip-path in SF Symbol output

[rursache]

Summary

Fixes #37

Pre-change, emitting an sfsymbol for an SVG that used <clipPath> printed clip-path unsupported in SF Symbols and produced no content. This PR bakes path ∩ clipPath into the emitted symbol path so the final SVG contains the already-clipped geometry (SF Symbols themselves still don't support clipPath, so the clip has to be resolved at generation time).

Approach

• Follows the existing Renderer.SFSymbol+CGPath.swift precedent: uses CoreGraphics from inside the SFSymbol renderer when it gives us hard geometry we'd otherwise have to implement from scratch, all behind #if canImport(CoreGraphics)
• New Renderer.SFSymbol+ClipPath.swift uses CGPath.intersection(_:using:) on macOS 13 / iOS 16 / tvOS 16 / watchOS 9 with a bbox-only fallback for older deployment targets so nothing crashes or silently corrupts output
• LayerTree.Layer.clip is no longer an early-reject in getSymbolPaths. After the recursion collects child paths the new applyClip runs each through the intersector. Entirely-outside shapes drop, entirely-inside shapes pass through, partial intersections become baked geometry
• Honors clip-rule (evenodd/nonzero) and per-child transform inside <clipPath>
• Adds clipPathUnits (userSpaceOnUse default + objectBoundingBox) by threading a new DOM.ClipPath.Units through the parser and a new LayerTree.ClipUnits on LayerTree.Layer. The SFSymbol clipper applies translate(bbox.x, bbox.y) * scale(bbox.w, bbox.h) when units are objectBoundingBox, computed against the path being clipped

What's covered

• <clipPath> with rect, circle, ellipse, polygon, path children
• Multiple children unioned inside a single clipPath
• clip-path on a <g> propagating to all descendants
• clip-rule="evenodd" producing hollowed regions
• clipPathUnits="objectBoundingBox" with shape-relative coordinates
• Per-child transform inside <clipPath>
• Composition with transform on the clipped element's parent
• Identity pass-through when the clip fully contains the shape
• Drop when the clip is entirely outside the shape

Tests

• 12 new SVG fixtures under SwiftDraw/Tests/Test.bundle/clip-*.svg covering every variant above
• New RendererSFSymbolClipPathTests with 18 cases:
  • The exact issue Missing support for clipPath #37 minimal SVG embedded inline, asserting non-empty paths in all three weight variants
  • One geometry test per fixture, comparing the intersected path.bounds against the analytically expected result with 0.5pt tolerance to absorb bezier approximation
  • Two LayerTree.Builder tests confirming clipUnits flows through from DOM for both default and objectBoundingBox
  • Full-pipeline tests through the SF Symbol template (counts paths per weight variant, asserts empty-result throws No valid content)

swift test: 214 passed, 0 failed (was 196 pre-fix, +18 new). No regressions in the pre-existing rasterization or sfsymbol tests.

Test plan

• swift test passes across the full package
• Pre-fix repro SVG (100x100 rect clipped by r=30 circle) now renders a symbol whose paths match the clipping circle
• Existing PNG rasterization of a clipPath-free SVG (Moonsama.svg linked in Missing support for clipPath #37) is unchanged
• Existing sfsymbol output for SVGs without clipPath is unchanged (chart.svg, dash.svg, key.svg tests all still pass)

[codecov]

Codecov Report

❌ Patch coverage is 95.49839% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.95%. Comparing base (8f03233) to head (54348dd).

Files with missing lines	Patch %	Lines
.../Sources/Renderer/Renderer.SFSymbol+ClipPath.swift	81.35%	11 Missing ⚠️
...ests/Renderer/Renderer.SFSymbolClipPathTests.swift	99.06%	2 Missing ⚠️
SwiftDraw/Sources/LayerTree/LayerTree.Layer.swift	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #124      +/-   ##
==========================================
+ Coverage   89.80%   89.95%   +0.14%     
==========================================
  Files         159      161       +2     
  Lines       15868    16174     +306     
==========================================
+ Hits        14251    14549     +298     
- Misses       1617     1625       +8     

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