Add generic markdown layout export helpers

[JSv4]

Summary

• Add JSON-friendly helpers for Markdown payloads, table regions, normalized table HTML inputs, and framework-neutral layout predictions.
• Fix box serialization so attribute-only, mapping, and indexable box objects all produce geometry consistently.
• Expose payload and layout-prediction convenience wrappers through the public ingestor API.
• Update internal adapter code to consume the shared engine-level layout output instead of duplicating table and geometry reconstruction.

Validation

• uv run make check
• Focused exporter and adapter tests
• git diff --check