Fix live loupe background refresh

[yvette-carlisle]

Summary

• sample live loupe patches from the same latest frame authority used for RGB sampling
• keep stream/window-list sampling as fallbacks so stationary background changes update without widening hot-path capture cost
• preserve the existing fast HUD/loupe render path and sample cache behavior

Validation

• cargo make lint-swift
• cargo make test-macos-native-host
• cargo make test-macos-native-host-stage
• HUD_FOLLOW_CASES=loupe PATH_DURATION_MS=800 PATH_CYCLES=1 POST_PATH_SETTLE_S=0.25 OVERLAY_SETTLE_S=0.2 POST_CLOSE_SETTLE_S=0.12 RSNAP_TELEMETRY_LAST=8s scripts/smoke/native-hud-follow-macos.sh
  • PASS; sample_refresh_gap p95=8.89ms against 120Hz sample budget 9.33ms
  • active_layer_chrome_render_gap p95=14.28ms against 60Hz budget 16.67ms