feat: opt in to record-oriented Markdown exports

[sks]

Summary

• Add opt-in record-oriented table output for Markdown exports. The default Markdown table format does not change.
• Support --markdown-table-format records in convert and convert-remote. Remote conversion still applies the option only when writing Markdown locally.
• Provide export_to_record_markdown() and save_as_record_markdown() for packaged SDK users. Keep image modes and other output formats unchanged.
• Remove the fork-specific Serve integration from PR feat: avoid padded-grid allocation in table exports #1. The serializer does not replace a process-wide Core serializer.

This branch is based on the current stackgenhq/main. Draft PR #2 contains the initial serializer. The branch comparison now shows only the 11-file opt-in addition, without unrelated upstream commits.

Checks

• Selected tests: 164 passed, 2 deselected (test_cli_convert), 91 warnings.
• make validate passed on the 11-file opt-in changeset.
• git diff --check passed.
• Full repository test suite and make check were not run for this PR.

This export option does not address conversion memory use or add a Docling Serve-wide option. A general Markdown serializer API remains a docling-core design decision.

• Documentation updated
• Tests added

[github-actions]

AI triage (advisory)

Checked commit 259b44f963. The analysis read the diff only. It did not run code from this pull request.

Warning

The AI analysis did not finish. Only the deterministic checks are shown.

Topic labels added: enhancement.

CI safety: not assessed.

Possibly related pull requests

PR	State	Signal	Assessment
#2	open	referenced as #2; changes the same code	not assessed
#1	merged	referenced as #1	not assessed

_{The ai:* labels are advisory. They do not approve CI and they do not add tests:full. A new push runs this check again.}