fix: validate receipt lineage parents

[sedge-bot]

Summary

• make receipt-log validation reject parent_receipt values that do not exist in the same log
• document that validate now checks lineage links, not only required fields and duplicate IDs
• add regression coverage for unknown parent receipts

Test Plan

• python3 -m unittest tools/receipt-log/test_receipt_log.py -v
• python3 tools/receipt-log/receipt_log.py validate
• python3 tools/receipt-log/receipt_log.py fingerprint --json + python3 -m json.tool
• python3 -m py_compile tools/receipt-log/receipt_log.py tools/receipt-log/test_receipt_log.py
• git diff --check
• python3 -m unittest discover -s tools/receipt-log -p 'test_*.py' -v
• python3 -m unittest discover -s tools/proposal-pile -p 'test_*.py' -v
• /opt/sedge/.local/bin/uv run --extra dev pytest -q (13 passed)

Refs #9