Derive device voltages from project settings

docs/circuit-list-editor-api.md

@@ -150,7 +150,11 @@ synchronizes linked circuit rows implicitly.
   - response:
     `{ "project": { ... }, "revision": { ... }, "history": { ... } }`
   - persistent Undo/Redo restores metadata, voltage values and the enabled
-    distribution-board supply types together
+    distribution-board supply types together; project-device and circuit
+    voltages are recalculated from the restored project settings in the same
+    transaction
+  - project-device and circuit voltage are derived values and are not accepted
+    as editable frontend fields
   - at least one of `AV`, `SV`, `EV`, `USV`, `MSR`, `SiBe` must be enabled;
     a supply type currently used by a distribution board cannot be disabled
   - unchanged values are rejected without creating a revision; a stale

docs/project-history-and-external-model-architecture.md

@@ -265,7 +265,9 @@ Completed foundation:
   snapshots in the same transaction
 - `project.update-settings` persists both project voltage defaults, its exact
   inverse, revision and history transition atomically; the project page and
-  existing PUT route require the current expected revision
+  existing PUT route require the current expected revision; the same
+  transaction derives every project-device and circuit voltage again, so
+  Undo/Redo cannot leave mixed voltage states
 - `distribution-board.insert` persists the complete generated board, circuit
   list and default-section structure with stable ids; its delete inverse
   refuses changed or populated structures and the project-page POST tracks the

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