[codex] Fix Simulator PhotosCrop black canvas

[muukii]

Summary

Fixes PhotosCrop rendering as a black canvas on iOS Simulator while keeping the 10-bit Display-P3 drawable path on real devices.

Root Cause

The editing canvas drawable pixel format was changed to .bgr10a2Unorm for the wide-gamut Display-P3 canvas path. That works on device, but recent iOS Simulator runtimes can create the MTKView and still present the drawable as black.

Changes

• Use .bgra8Unorm for the editing canvas drawable on Simulator builds.
• Keep .bgr10a2Unorm for device builds.
• Add a regression test that locks the intended platform-specific drawable format while preserving the existing wide-gamut color math tests.

Verification

• Reproduced the black PhotosCrop canvas in SwiftUIDemo on iPhone 17 Pro, iOS 26.5 Simulator before the fix.
• Verified the same PhotosCrop Horizontal screen renders the image after the fix.
• Ran BrightroomEngineTests/EditingCanvasColorContractTests: 3 passed.
• Built and launched SwiftUIDemo successfully on the same Simulator.