Improvement: Add Windows CI and expand test coverage

## Summary

Waiter has good CI trigger configuration (push + PR) but is missing a Windows CI workflow and has minimal test coverage.

## Current State

- **CI workflows:** `docc.yml`, `macOS.yml`, `ubuntu.yml` — missing `windows.yml` (present in most other 0xLeif packages)
- **Test coverage:** Only 1 test file (`WaiterTests.swift`) covering 2 source files (`Wait.swift`, `Waitable.swift`)
- **CI triggers:** Well configured (both push and PR triggers) ✓
- **actions/checkout:** Already on v4 ✓

## Suggested Improvements

1. **Add `windows.yml` workflow** — modeled after the equivalent workflow in AppState/Later/Fork/Cache
2. **Expand test coverage** — add tests for edge cases, cancellation behavior, timeout scenarios, and the `Waitable` protocol

## Priority

Test coverage expansion is the highest impact improvement. The Waiter API has interesting concurrency semantics (waiting with conditions) that would benefit from thorough testing of edge cases.

🤖 Filed by corvid-agent during autonomous improvement pass