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phases/5-markdown-rendering-assets/step3.md

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+# Step 3: chunk-renderer
+
+## Read First
+- /AGENTS.md
+- /PLAN.md
+- /PROGRESS.md
+- /docs/HARNESS.md
+- /docs/IMPLEMENTATION_PLAN.md
+- /docs/ARCHITECTURE.md
+- /docs/CONVERSION_POLICY.md
+- /phases/1-core-runtime-contracts/step2.md
+- /phases/5-markdown-rendering-assets/step2.md
+
+## Task
+Implement chunk planning and chunk Markdown bundle writing over enriched blocks.
+
+Chunk boundaries should target 20 pages but preserve logical block integrity for paragraphs, tables, figures, and formulas.
+
+## Sprint Contract
+- Done means: chunk files with frontmatter can be written deterministically from internal document fixtures.
+- Hard thresholds: block integrity is preserved at chunk boundaries; chunk frontmatter includes minimum context; quality gates run on rendered chunks.
+- Files owned: `src/pdftomd/chunking.py`, `src/pdftomd/renderer.py`, tests, `PROGRESS.md`, phase index.
+- Dependencies: Renderer, assets, and output bundle contracts.
+
+## Acceptance Criteria
+```powershell
+python scripts\validate_workspace.py
+.\venv\python.exe -m pytest tests
+```
+
+## Verification
+1. Run the acceptance commands.
+2. Confirm long-document chunk fixtures cover boundary behavior.
+3. Update `PROGRESS.md` and this phase index.
+
+## Do Not
+- Do not split blocks in the middle to satisfy exact 20-page counts.
+- Do not create document sidecar metadata files.
+- Do not implement CLI orchestration here.