Fix HUD scrim overlap masking

[yvette-carlisle]

Summary

• keep the live scrim focus hole separate from HUD/loupe rounded exclusions so overlapping holes do not double-toggle back into scrim
• preserve rounded HUD/loupe scrim exclusions through a layer mask for the live renderer
• add a native-host probe covering the partial focus/HUD overlap regression

Validation

• cargo make test-macos-native-host
• cargo make fmt-swift-check
• cargo make lint-swift
• git diff --check
• launched the signed app with ./scripts/build_and_run.sh run for local visual testing