Add MSI direct UDP retrieval foundation

[stevenboyd78]

Summary

• promote exact one-shot MSI retrieval into the shared SDS200 bounded-XML command map and direct UDP expectation/reassembly/retry machinery
• allow SDSScanner.get_msi() only for the repository's direct UdpTransport path, including capture recording, while keeping fallback and arbitrary UDP-like transports fail-closed
• add deterministic fake-datagram coverage for bare MSI, exact request matching, numbered fragments, exact-wire retry, state-neutral retrieval, and capture-wrapper behavior
• update roadmap/research/fixture provenance to keep physical scanner, firmware, model, unindexed MNU, and MSV/MSB behavior explicitly deferred

Validation

• ruff check .
• mypy src/sds200 — 137 source files
• python -m pytest — 3617 passed
• python scripts/check_docs.py — 47 Markdown files
• package wheel/sdist build
• twine check dist/*
• exact feature-SHA push CI run and all five check runs completed successfully

Evidence boundary

This slice uses deterministic software-only UDP fixtures. It does not claim physical SDS200 UDP behavior, firmware/model applicability, or establish MSV/MSB [RSV] serialization. Fallback MSI and unindexed MNU remain deferred.