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

## 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