Unify tutorial: update browser overlay tests #766

Summary

Update tests/main-street/TutorialOverlayManager.browser.test.ts and tests/main-street/TutorialOverlayHighlights.browser.test.ts to cover unified rendering. The unified MainStreetTutorialHints.showStep() method must render correctly with the merged step set and action gate logic.

Acceptance Criteria

• Browser tests verify overlay rendering for all 13 unified steps
• Highlight zone tests cover all SLL-defined zones in the unified system
• Screenshot regression tests pass for:
  • HUD highlight
  • Market rows highlight
  • Street grid highlight
  • Incident queue highlight
  • End turn button highlight
  • Investments row highlight
  • Help button highlight
  • Center modal (no highlight)
  • Completion modal (no highlight)
• DOM tooltip path works in browser test environment
• Canvas fallback path is tested (verify fallback logic exists)

Dependencies

• None (browser tests are independent, but must exist before unified rendering is implemented)