Week 1 — Relativistic-ready Boris & Test Scaffolding

Deliverables - Boris pusher (Python) + analytic helix helpers - Tests: speed conservation, radius check; EM energy closure - I/O stubs (ROOT/HepMC3 fall back to npz/ascii)

Exit criteria - pytest -q green - Demo spiral stable with dt guard

Run ```bash cd pt-sim && pip install -e . pytest -q