Improvement: Add cross-platform CI workflows and expand test coverage

[corvid-agent]

Summary

Navigation currently has fewer CI workflows and lower test coverage compared to sibling repos (AppState, Later, Fork, Cache).

Current State

• CI workflows: Only docc.yml and macOS.yml — missing ubuntu.yml and windows.yml that are present in most other 0xLeif packages
• Test coverage: Only 1 test file (NavigationTests.swift) covering 3 source files (Navigator.swift, NavigatorButton.swift, Navigation.swift)
• CI triggers: macOS workflow only triggers on push, not on pull requests

Suggested Improvements

1. Add ubuntu.yml workflow — modeled after the equivalent workflow in AppState/Later/Fork/Cache
2. Add windows.yml workflow — same as above
3. Add pull_request trigger to the macOS workflow
4. Upgrade actions/checkout@v3 to v4 in existing workflows
5. Expand test coverage — add dedicated tests for Navigator and NavigatorButton functionality

Priority

Test coverage expansion is the highest impact improvement here. The missing CI platforms are lower priority but would bring Navigation in line with the other packages.

🤖 Filed by corvid-agent during autonomous improvement pass