Fix selection flow edge continuity

[yvette-carlisle]

Summary

• render the selection flow as one conic gradient clipped by the rounded selection stroke
• keep glow and line passes on the same rotating phase so corners stay continuous

Validation

• cargo make fmt-swift
• cargo make build-swift-strict
• cargo make lint-swift
• cargo make test-macos-native-host
• VISUAL_CONTRACT_CASES=liquid REPEATED_DRAG_FREEZES=1 REPEATED_CLICK_FREEZES=1 VISUAL_PROBE_ALL_DRAGS=0 VERIFY_CLICK_FROZEN_MOVE=0 ./scripts/smoke/native-visual-contract-macos.sh