- normal_form.plot_mixed: the p* equilibrium label sat on top of the crossing
  lines; move it clear of the intersection and add a translucent white bbox so
  it stays legible.
- Add hlines/vlines reference-line kwargs to NormalFormGame.sweep_mixed/sweep_pure
  and ExtensiveFormGame.sweep_mixed/sweep_pure (notebook API parity; fixes
  "sweep_pure() got an unexpected keyword argument 'vlines'").
- Promote ref_lines into viz/plots.py (shared) and fix the coincident NE-indicator
  over-paint in ExtensiveFormGame.sweep_pure (same fan-out as normal_form).

92 tests pass, pyflakes clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>