Derive device voltages from project settings

docs/spec/08-current-product-backlog.md

@@ -35,21 +35,27 @@ requirements and intended sequencing, not proof of implementation.
 - Persist changes through project commands so they remain undoable after a
   restart.
 
-## Device Voltage Defaults
+## Device Voltage Derivation
 
 - Derive voltage from the selected phase type and the project settings:
   - single-phase uses the project's single-phase voltage;
   - three-phase uses the project's three-phase voltage.
-- Apply this default consistently when creating global devices, project
-  devices and manual circuit-list entries.
-- Define explicit override behavior before implementation so a user-entered
-  exceptional voltage is not silently overwritten when phase type or project
-  settings change.
+- Voltage is not editable on global devices, project devices or circuits.
+  Global devices store only their phase; copying one into a project derives
+  the target project's voltage.
+- Project-device voltage follows its phase. Circuit voltage follows the
+  section phase; an unassigned circuit is three-phase only when all assigned
+  device rows with a valid phase are three-phase.
+- Changing project voltage settings updates all project devices and circuits
+  atomically in the same persistent Undo/Redo step.
+- Database migration `0019` and snapshot schema version `5` normalize older
+  stored values. Version-four and older imports remain supported and are
+  normalized while being upgraded.
 
 ## Recommended Sequence
 
 1. Distribution-board floor and supply fields.
-2. Device voltage default and override semantics.
+2. Device voltage derivation without overrides.
 3. Snapshot-to-revision descriptions and history presentation.
 
 The Revit/CSV/IFCGUID round-trip remains a separate jointly planned phase and