Reverse conversion: docx → clean md (Tier 2)

[llbbl]

For Tier-2 reverse-import consumers (polydoc). Lower priority — TeamWiki does NOT use reverse import.

Tasks

• docx → clean md via pandoc (-f docx -t gfm) or a mammoth fallback.
• Run through the optional postprocessor hook.
• Golden-file tests + a full md → docx → md round-trip (with a consumer-provided sidecar restoring lossy bits).

Acceptance: docx round-trips to clean md; unmappable items are surfaced, never silently dropped.

Depends on: "Conversion core: md → docx (pandoc + reference.docx)" · prioritize after the transports.