Accept calendar-month label horizons

[stefan-jansen]

Closes #60

Summary

• accept 1M, P1M, and integer multiples as label horizons
• normalize each month to the documented fixed 30-day duration required by purge arithmetic
• improve invalid-duration guidance and document the approximation

Verification

• reproduced the prior WalkForwardConfig(label_horizon="1M") validation failure
• uv run pytest tests/test_splitters/test_config.py tests/test_splitters/test_walk_forward.py tests/test_splitters/test_calendar_first.py -q -n 0 (83 passed)
• uv run ty check
• pre-commit run --all-files

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.