feat: implement md-to-word-swift converter

[PsychQuantClaw]

Summary

md-to-word-swift needs a follow-up implementation pass because the package exists on main, but the tracked source/tests are missing. We need the direct Markdown → Word (.docx) converter restored as a proper Layer 3 package.

Requirements

• Implement md-to-word-swift as a direct Markdown AST → OOXML .docx converter
• Preserve headings, paragraphs, emphasis, links, lists, code blocks, tables, and frontmatter-derived document metadata
• Keep the package independent and compatible with the existing MacDocCLI command surface

Layer 1 / dependency plan

• swift-markdown for Markdown AST parsing
• ooxml-swift for .docx generation
• doc-converter-swift for DocumentConverter / StreamingOutput protocol integration

Implementation shape

• Add a DocumentConverter conformer for Markdown → Word output
• Emit .docx bytes through the streaming architecture instead of routing through another converter
• Wire the package into MacDocCLI with the existing markdown → word command path

Test strategy

• Package-level unit tests for heading/list/table/link/code-block/frontmatter mapping
• Smoke test that generates a .docx and validates key OOXML/document metadata round-trips
• Keep package-local coverage above 80%