Task: Add unit tests and complete the Brave/macOS regression matrix

[Millbot-AI]

Goal

Cover the gesture and navigation decisions with automated tests plus a reproducible manual Brave/macOS matrix.

Acceptance criteria

• Test gesture thresholds and direction mapping.
• Test momentum deduplication.
• Test internal-history versus opener decisions.
• Test invalid and closed opener tabs.
• Test nested opener relationships.
• Run the documented manual matrix for vertical scroll, horizontal containers, and native navigation.
• Record browser and macOS versions with the results.