Fix live HUD overlay masking

[yvette-carlisle]

Summary

• reuse rounded overlay mask geometry for live/frozen scrim exclusions without permanent or rectangular holes
• keep live HUD/loupe from being darkened by the live scrim while filtering scrim redraws to changed visible exclusions
• restore straight selection-flow corners with a low-cost square flow mask plus static corner accents for the small cross/# feel
• add native host pixel probes for rounded scrim and mask exclusions

Verification

• cargo make fmt-swift
• cargo make test-macos-native-host
• cargo make lint-swift
• RSNAP_NATIVE_HOST_FORCE_REBUILD=1 HUD_FOLLOW_CASES=hud PATH_DURATION_MS=2500 PATH_RATE_HZ=120 PATH_CYCLES=3 OVERLAY_SETTLE_S=0.35 POST_PATH_SETTLE_S=0.6 POST_CLOSE_SETTLE_S=0.25 scripts/smoke/native-hud-follow-macos.sh (PASS, runID 9D0B116A-B50B-416E-91E4-6BEFF52DEFC0)