Feature: Detect internal back navigation, including SPAs

[Millbot-AI]

Goal

Distinguish a child tab that can meaningfully navigate back internally from one that has returned to its entry page.

Acceptance criteria

• Do not use history.length as the sole source of truth.
• Cover full-page navigations and same-document SPA navigation.
• Navigate inside the current tab before considering opener-tab behavior.
• Treat incomplete or contradictory state as a safe no-op.
• Document the chosen signals and their known limitations.
• Add focused tests for multiple back steps and SPA routes.