Summary

Extracts the effective section-ownership rollup out of the larger feat/doc-comment-inbox branch into a standalone change off main, to shrink the inbox PR. Infra-only.

The siteIndex worker already materializes the dense sections registry from RwSite.listSections(). This adds a rollup layer: the worker loads per-site ownership claims (written to section_ownership by the scan) and folds an effective owner into every section row.

• migration: add entity_ref / entity_owner_ref + sections_owner_idx to sections (edited in place — siteIndex is unreleased)
• effectiveOwnership.ts: pure computeSectionRows — attributes each section to its nearest claiming ancestor, falls back to the site-root sentinel then a null owner, and strips the claimer's path prefix so section_path is owner-relative
• SectionOwnershipStore.listForSite: read side for the worker
• runWorker: fetch claims alongside listSections/listPages and roll them up
• plugin.ts: pass the existing sectionOwnershipStore into the worker

Scope

No consumer reads the new columns yet — the comment inbox (a separate change) is the first reader. This is deliberate pre-positioning.

Known deferred debt

The scan (writes claims) and worker (reads claims) couple through section_ownership with no read-time consistency guard. Bounded and self-healing within ~one scan cycle, since ownership is part of the hashed SectionRow. The inbox PR (first reader) should revisit before adding generation plumbing.

Testing

• effectiveOwnership unit tests (nearest-ancestor, sentinel fallback, null-owner, path-boundary stripping)
• SectionOwnershipStore.listForSite, runWorker rollup + hash-stability, migration schema test
• Full rw-backend suite: 138/138 passing; typecheck + lint clean
• Verified live via yarn dev: 3 claims rolled up into 1368 attributed sections, owner-relative paths, no errors

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