feat: WP-0004 T01-T04 — stable corpus, ADRs, regression test

- corpus/markidocx-docs/manifest.yaml: specs as live markidocx project (FR-1101)
- corpus/markidocx-docs/known-drift.md: documented structural drift
- workflows.py: release-regression accepts manifest path; emits corpus_id (FR-1109)
- tests/regression/test_corpus_regression.py: corpus regression suite (FR-1102–1110)
- architecture/ADR-002: python-docx as conversion engine
- architecture/ADR-003: manifest YAML schema
- workplans/MRKD-WP-0004: T01–T04 done; T05 blocked (SBOM path mapping needed)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -7,6 +7,9 @@ ralph-workplan/
 # Runtime evidence store (generated by evidence.py during builds/tests)
 .markidocx/
 
+# Corpus build artefacts (generated DOCX and imported Markdown)
+corpus/markidocx-docs/dist/
+
 # ---> Python
 # Byte-compiled / optimized / DLL files
 __pycache__/