Content pipeline subtree rules

This crate owns UI-independent asset, import, catalog, transaction, watcher, and processing behavior.
Preserve transaction atomicity, byte-identical rollback, stable identity, reference repair, derived-artifact ownership, managed-path protection, watcher suppression, and deterministic restart behavior.
Prefer focused tests here over heavy editor tests.
Never depend on the editor crate.
Keep GPU and rendering requirements out of headless paths.
A defect in one content operation may justify one bounded sibling-operation invariant audit, not an unlimited repository audit.