Add lightweight Windows and macOS compatibility smoke jobs

[bagowix]

Motivation

The package declares generic Python 3.11–3.14 support, but CI executes only on Ubuntu. Platform-sensitive areas include:

• threading.Timer, daemon workers, and sync timeout behavior;
• loopback HTTP E2E tests and socket cleanup;
• asyncio cancellation and event-loop shutdown;
• path/package installation behavior.

A foundational library benefits from detecting platform regressions before users do.

Proposed CI shape

Keep the full quality/coverage/Redis matrix on Ubuntu. Add a smaller smoke matrix on windows-latest and macos-latest for the oldest and newest supported Python versions, or start with one Python version per OS if runtime cost is a concern.

Acceptance criteria

• Run core, timeout, pipeline, state-machine, typing-surface runtime, and loopback integration tests on Windows and macOS.
• Skip only tests that genuinely require Linux services; document every exclusion.
• Use uv sync --frozen and the same locked dependency graph.
• Ensure jobs are cancellable with the existing workflow concurrency policy.
• Keep the matrix intentionally lightweight and avoid duplicating Ruff/mypy/pyright on every OS.
• Document the supported/tested platform policy in CONTRIBUTING.md or the docs.