Docs: add a copy-paste document-report adapter lock example

[bitjaru]

Why

Roadmap issue #9 will define the full public adapter specification. The existing document-report profile explains the principles but lacks the copy-paste lock example already available for social-carousel.

Scope

Add one concrete document-report lock example to engine/ADAPTERS.md that declares:

• artifact type and paper size
• margins, header/footer, and reading measure
• editable source and PDF export
• page-by-page render evidence
• widows/orphans, clipped tables, links, page numbers, and accessibility checks

Add one short failing example showing a web-layout assumption that should not cross into a paginated report.

Acceptance criteria

• The example follows the existing adapter authority boundary: StyleSeed owns judgment; the renderer owns physical output constraints.
• It is renderer-neutral and does not require one commercial document tool.
• It does not invent a new adapter schema or close the parent specification issue.
• Run node demo-pricing/scripts/build-llms.mjs, node scripts/validate-engine.mjs, and git diff --check.
• Commit any regenerated mirrors.

Link the PR to #9 without closing #9. New contributors are welcome to claim this issue by leaving a comment.