Add deterministic Pandoc conversion core

[llbbl]

Summary

• add a typed Pandoc 3.x conversion surface for deterministic Markdown-to-DOCX output
• cover conversion behavior with unit tests, a normalized golden XML fixture, and a reference DOCX fixture
• document the public API, reproducibility contract, and Pandoc expectations for consumers

Tests

• pnpm format:check
• pnpm lint
• pnpm typecheck
• pnpm test (19 passed, 1 expected skipped)
• pnpm build
• pnpm pack --dry-run
• built a real Pandoc 3.10 conversion
• pnpm audit

Design notes

• SOURCE_DATE_EPOCH is forwarded to Pandoc so identical inputs can produce reproducible DOCX bytes.
• --wrap=none and --markdown-headings=atx are intentionally omitted because they affect Markdown output, not DOCX writing.

Residual risks

• Byte-for-byte reproducibility still depends on the installed Pandoc binary and the reference DOCX remaining stable.

Closes #2