docs: add tamper verification walkthrough

[vsolano9]

Summary

• add a runnable, service-free tamper walkthrough for source, citation, Merkle-root, and signer-key changes
• document the exact verifier output and explain why key pinning matters after re-signing
• lock the README transcript to the executable output with an end-to-end regression test

Validation

• uv run --python 3.11 pytest -q (86 passed)
• uv run --python 3.12 pytest -q (86 passed)
• uv run --python 3.11 ruff check .
• uv run --python 3.12 ruff check .
• focused tamper test repeated 20 times on Python 3.11 and 3.12
• built wheel and sdist; installed wheel into a clean Python 3.12 environment and ran the packaged example plus CLI smoke test

Closes #4