Sprint 5M: DOCX artifact parsing v0

[samrusani]

Summary

• add narrow DOCX ingestion on the existing artifact-ingestion seam
• preserve retrieval and compile contracts while persisting deterministic chunk rows
• add unit and integration coverage and update build/review/architecture docs

Verification

• ./.venv/bin/python -m pytest tests/unit
• ./.venv/bin/python -m pytest tests/integration
• ./.venv/bin/python -m pytest tests/unit/test_semantic_retrieval.py

Review

• REVIEW_REPORT.md verdict: PASS
• Merge authorized in chat