Port rendering to Metal and add comprehensive test suite

[slarson]

Summary

• Fix Metal rendering pipeline (missing setFragmentBuffer for fragment shader)
• Implement staged opacity for layer peeling (cuticle→organs→muscles→neurons)
• Fix initial camera zoom to properly fill screen
• Add fallback colors for missing materials
• Add fallback OWDraw creation when bounding boxes missing

Test Coverage (107 tests)

Test Suite	Tests	Description
OWVectorTests	13	Math/vector operations
OWInterpolantTests	20	Animation tweening
OWNavigateTests	33	Camera/navigation
OWResourceTests	41	Mesh/metadata loading

Test Plan

• [x] All 107 unit tests passing
• [x] App builds on iOS Simulator
• [x] Worm renders with proper colors
• [x] Opacity slider peels layers correctly
• [x] Pinch/pan gestures work