Update ORBDisplayList API and add encoding protocols

[Kyle-Ye]

Summary

• Add full ORBDisplayList API surface (properties, drawing, filters, styles, layers, transforms)
• Add ORBDisplayListContents and ORBDisplayListInterpolator protocols/classes
• Add encoding protocols: ORBEncodable, ORBDecodable, ORBEncoderDelegate, ORBDecoderDelegate, ORBEncoderSet
• Move _ORBDrawableDelegate.h and add _ORBDisplayListContents.h to Render/Private/
• Add CF_RETURNS_NOT_RETAINED to beginCGContextWithAlpha: for clean Swift bridging
• Update OpenRenderBoxShims typealiases and DarwinPrivateFrameworks dependency

Test plan

• Compatibility tests pass (DisplayListTests: init, properties, beginEndCGContext, moveContents, NSImage)
• Verify swift build on non-Darwin platforms

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 159 lines in your changes missing coverage. Please review.
✅ Project coverage is 0.00%. Comparing base (9a04f86) to head (dacf2da).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
Sources/OpenRenderBox/Render/ORBDisplayList.swift	0.00%	138 Missing ⚠️
...nRenderBox/Render/ORBDisplayListInterpolator.swift	0.00%	21 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##            main     #25    +/-   ##
======================================
  Coverage   0.00%   0.00%            
======================================
  Files          6       8     +2     
  Lines        197     356   +159     
======================================
- Misses       197     356   +159     

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