Generate a paper-first empirical draft from structured research and estimation artifacts.

Use this after a research brief and baseline econometric outputs already exist. The tool only consumes saved structured artifacts and must not invent unsupported numbers.

Outputs:
- `paper_draft.md`
- `paper_draft.json`
- `paper_appendix_outline.md`
- `paper_asset_manifest.json`

Rules:
- Numeric claims must be traceable to structured result artifacts from the same run.
- If heterogeneity or mechanism artifacts are missing, mark those sections as `待补充`.
- Do not fabricate references.
