Docs: tamper example, quickstart output, unknown-source edge test

[AshSgDe29071999]

Summary

Addresses the first-run and “what does failure look like” gaps, plus one real verifier edge case.

Fixes #4 — what a tampered receipt looks like

• New examples/tamper.py builds a receipt then walks each failure mode from tests/test_tamper.py:
  1. sources — source content edited after signing
  2. grounding — citation swapped to an unknown source id
  3. merkle — merkle root edited by hand
  4. re-sign — payload edited and re-signed with a different key (signature passes; signer_pin catches it)
• Representative output pasted into the README under What failure looks like.

Fixes #1 — quickstart

• Quickstart now has install, the minimal snippet, and an expected verification output block so copy-paste is self-checking.

Fixes #2 — untested edge case

• verify_receipt(..., source_contents={..., "never-retrieved": "..."}) must fail the sources check with an explicit unknown source detail rather than being silently ignored.
• Covered in tests/test_verifier.py::test_unknown_source_id_in_contents_fails_sources_check.

Design choices

• Tamper demo mirrors test fixtures (same SOURCES/ANSWER as conftest.py) so printed checks match what pytest already asserts.
• Re-sign case is split into 4a (no pin → valid) and 4b (pin original key → fail) because that is the non-obvious threat-model point.
• Edge test targets the API contract of verify_sources, not a trivial empty-input fluff case.

Test plan

• pytest -q
• python examples/tamper.py
• ruff check .