Editor subtree rules

Define an explicit interaction-state matrix before implementing new editor UI behavior.
Keep egui rendering thin. Move selection models, transaction planning, path rewriting, material resolution, import review, and reusable logic into lightweight modules or crates.
Test an invariant in its lightest owning crate; do not put it in editor merely because the UI calls it.
A small UI change does not justify full-workspace testing.
Use cargo check -p editor --lib through the development lane in the fast loop unless a binary target is directly affected.
Use focused editor tests only for editor-owned behavior.
Use named native scenarios for interaction and visual acceptance.
Do not use all-features or hot-reload/dynamic-linking combinations for ordinary editor QA unless that feature is under test.
Screenshots alone do not establish UX completion; exercise the interaction.
Reuse established editor components and visual language instead of creating bespoke panel controls.
Treat sustainable extension seams and debt-at-the-point-of-change as a feature requirement. Inspector and Content Browser shells dispatch; they do not own domain behavior. Do not grow a frozen-baseline module, and run scripts/codex/architecture_audit.py check after structural UI changes.