docs: draft parsed result ZIP contract RFC

[Frankie-Xu]

Related to #45. RFC only — no models, validator, or worker change.

Summary

• Add docs/design/parsed-result-contract.md, a draft contract for the parse-result ZIP (chunks.json, doc_nav.json, manifest.json).
• Answers nuemaan's three scope questions from 2026-06-12 (schema location, hard-fail vs warn, versioning) against current writers on main @ 9489fa2c.
• Locks the observed v1 shape, including chunk_type=page and table content as an asset path, which AGENTS.md does not fully describe.
• Asks maintainers to confirm five decisions on Parsed result contract #45 before any implementation PR.

Verification

• Docs-only. No Python change.
• Rechecked writers: zip_package_writer.py, zip_result_service.py, zip_chunk_schema.py, zip_doc_navigation.py, zip_manifest_schema.py, finalize_parse_success.
• test_parse_task_contract.py still only asserts file presence + counts; this RFC does not extend it.

Deployment Notes

• No environment variables, migrations, or runtime change
• Rollback is reverting this PR

Checklist

• Tests were added or updated when behavior changed — N/A (RFC only)
• Public docs, examples, or OpenAPI contracts were updated when needed
• Database migrations are idempotent and safe to deploy — N/A
• Logs, errors, and validation paths avoid leaking secrets or user data
• The pull request description explains any breaking or user-visible change — none; no code