Add cross-platform tests and package CI

[rossant]

Summary

• run the full phylib test suite on Linux (Python 3.10-3.12) and macOS
• run a stable electrode/statistics/utilities smoke suite on Windows
• build and upload release artifacts after the test matrix passes
• include the license and runtime requirements in source distributions

Why

phylib currently gives pull requests no automated test signal. The package build gate also exposed that wheels could not be built from the source distribution because requirements.txt was omitted.

The first run additionally confirmed broad pre-existing Windows I/O teardown failures caused by open memory maps. Those are outside this focused CI change, so Windows begins with the 60-test subsystem baseline while Linux and macOS run all 273 tests.

Impact

Pull requests and pushes to master receive multi-version and multi-platform test coverage plus a releasable-artifact check. Existing repository-wide flake8 debt is deliberately not introduced as a blocking gate.

Validation

• full suite: 273 passed locally
• Windows smoke target: 60 passed locally
• workflow YAML parsed successfully
• python -m build produced both sdist and wheel