Normalize SF Symbol variant paths for interpolation compatibility

[rursache]

Summary

• Fixes "the provided variants are not interpolatable" error when generating SF Symbols with separate weight variant SVGs
• Root cause: source SVGs (e.g. from Apple's CoreSVG) can have incompatible path structures between weights - one variant may have a cubic curve (rounded corner) where another has a line (sharp corner), producing different segment counts
• Adds two-phase path normalization after variant generation:
  1. Segment count alignment: inserts degenerate zero-length cubics in shorter variants to match the longest
  2. Type promotion: promotes L (line) to degenerate C (cubic) where types differ at the same position

Example (from the reported issue):

Before fix:

• Ultralight Path 1: 27 segments (14 cubics)
• Regular Path 1: 26 segments (13 cubics)
• Black Path 1: 26 segments (13 cubics)

After fix:

• All variants: 27 segments (14 cubics) with matching command sequences

Test plan

• Added testNormalizeSegments_PromotesLineToCubic - verifies L→C promotion
• Added testNormalizeSegments_InsertsDegenerate - verifies alignment with extra segments
• Added testNormalizeSegments_AlreadyMatching - verifies no-op when paths match
• Added testCommandType and testPromoteToCubic unit tests for helpers
• Verified with reporter's photo.error SVGs - all 3 variants produce matching segment sequences
• All 193+ tests pass, existing key.svg symbol test unaffected

Closes #57

[codecov]

Codecov Report

❌ Patch coverage is 93.88646% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.46%. Comparing base (6238058) to head (280336c).
⚠️ Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
SwiftDraw/Sources/Renderer/Renderer.SFSymbol.swift	90.62%	12 Missing ⚠️
...ftDraw/Tests/Renderer/Renderer.SFSymbolTests.swift	98.01%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #123      +/-   ##
==========================================
+ Coverage   89.02%   89.46%   +0.44%     
==========================================
  Files         159      159              
  Lines       11959    15618    +3659     
==========================================
+ Hits        10646    13973    +3327     
- Misses       1313     1645     +332     

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[swhitty]

wow thank you for this