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>

corpus/markidocx-docs/known-drift.md

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+# Known Drift — markidocx-docs Corpus
+
+Last updated: 2026-03-16
+
+## Summary
+
+The markidocx-docs corpus (PRD + FRS v0.2 + UCC) produces known structural drift
+on round-trip at LEVEL1. This drift is expected and does not indicate a regression.
+
+## Import mode: fallback (merged)
+
+The three source files are composed into a single DOCX. On import the system attempts
+to redistribute content back to the three origin files using source-boundary markers.
+The current build pipeline embeds section markers but the 27 H1-level sections in the
+combined document make boundary matching ambiguous, so the importer falls back to a
+single merged output (`dist/imported_merged.md`).
+
+**Classification:** expected / by-design. The merged output is complete and usable.
+
+## Structural drift items
+
+### Bold inline text in list items (broken: ~70 items)
+
+List items containing `**bold**` inline spans lose the bold markers on round-trip.
+python-docx represents inline bold as a `Run` with `bold=True`, but the importer's
+list-item text extractor concatenates run text without restoring markdown bold syntax.
+
+**Classification:** known limitation of LEVEL1 inline formatting in list items.
+**FR reference:** FR-508 (unsupported construct visibility) — these are surfaced
+explicitly as `broken` rather than silently accepted.
+**Impact:** content is preserved, presentation marker is lost.
+
+### Table (broken: 1 of 1)
+
+One table in the UCC is detected as missing after round-trip. Likely cause: the table
+contains merged cells or a header row structure that the importer does not reconstruct.
+
+**Classification:** known LEVEL1 table limitation.
+**Impact:** table content is present in the DOCX but not re-imported to Markdown.
+
+## Verdict
+
+902 elements preserved; ~71 broken items (all inline formatting in lists or 1 table).
+This corpus is suitable as a regression baseline: a clean round-trip regression test
+can assert `preserved >= 900` and `broken <= 80` rather than exact zero-drift.

corpus/markidocx-docs/manifest.yaml

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+project:
+  name: markidocx-docs
+  feature_level: level1
+  family: article
+
+sources:
+  - path: ../../specs/MarkiDocxProductRequirementsDocument_v0.1.md
+  - path: ../../specs/MarkiDocxFunctionalRequirementsSpecification_v0.2.md
+  - path: ../../specs/MarkiDocxUseCaseCatalog_v0.1.md
+
+output:
+  dir: ./dist
+
+metadata:
+  title: markidocx — Product Documentation
+  author: Markitect Project
+  date: "2026-03-16"