Extract circuit grid model

docs/circuit-list-editor-architecture.md

@@ -4,6 +4,15 @@
 
 The circuit-list editor is a circuit-first planning workspace for distribution-board design. It is optimized for spreadsheet-like editing while preserving electrical domain boundaries: circuit-level data stays on circuits, and load rows stay inside circuits.
 
+## Frontend Grid Modules
+
+- `circuit-tree-editor.tsx` owns React state, API commands, undo/redo and drag-and-drop orchestration.
+- `circuit-grid-model.ts` owns column metadata, circuit/device cell ownership, value projection, formatting, numeric parsing and block sort values.
+- `circuit-grid-insertion.ts` resolves insertion intent and insertion sort positions.
+- `circuit-grid-safety.ts` resolves delete intent, BMK conflicts and cross-section move confirmation requirements.
+
+The pure grid modules have no React state and are covered by focused unit tests. This keeps circuit/device ownership rules testable while the editor UI is split incrementally.
+
 ## Domain Model Overview
 
 - `CircuitSection`