Add Python 3.13/3.14 compatibility hardening and WAV guardrail tests

[erodola]

Summary

This PR finalizes Python compatibility hardening across supported versions and adds explicit guardrails around WAV sample I/O behavior.

Compatibility/runtime changes

• Expand declared support to Python 3.10 through 3.14.
• Add conditional dependency:
  • audioop-lts; python_version >= 3.13
• Avoid import-time failures on 3.13+ by moving pydub imports to WAV-only code paths in:
  • src/nodmod/modsong.py
  • src/nodmod/xmsong.py
• Add clear ImportError messages when WAV dependencies are missing.

CI + docs

• Expand CI matrix to run tests on 3.10, 3.11, 3.12, 3.13, 3.14.
• Update README requirements section to reflect supported versions and marker-based dependency behavior.

New tests

• Add lazy-import guardrails:
  • tests/test_python_compat_guardrails.py
  • Verifies import nodmod does not require pydub at module import time.
  • Verifies WAV APIs fail with clear scoped errors when pydub is unavailable.
• Add fixture-hash WAV stability checks:
  • tests/test_wav_fixture_hashes.py
  • Verifies deterministic sample load/save hashes for XM and MOD paths.

Validation

• Local suite:
  • uv run --with pytest pytest -q tests
  • 155 passed
• Full matrix in isolated envs:
  • Python 3.10: 155 passed
  • Python 3.11: 155 passed
  • Python 3.12: 155 passed
  • Python 3.13: 155 passed
  • Python 3.14: 155 passed
• Fresh install checks:
  • 3.12: audioop-lts not installed (expected)
  • 3.13: audioop-lts installed automatically (expected)
• Cross-version WAV probe (3.10-3.14): consistent waveform/frame hashes observed for tested fixture.

Notes

I closed issue #30 after implementing and validating the compatibility work. This PR is ready for your final review/merge checks.