Align documentation with clean baseline

docs/spec/03-data-model-concept.md

@@ -6,9 +6,10 @@ This is a conceptual model for Codex. It is not a final database schema, but the
 
 A distribution board contains a circuit list.
 
-A circuit list contains sections.
+A circuit list contains circuit groups. `CircuitSection` is the persisted name
+of that group aggregate.
 
-A section contains circuits.
+A circuit group contains circuits of one category.
 
 A circuit contains zero, one or multiple device rows.
 
@@ -60,29 +61,28 @@ User-facing fields:
 - `section`
 - `displayName` optional
 - `circuitTotalPower`
-- `protectionType`
-- `protectionRatedCurrent`
-- `protectionCharacteristic`
+- `protectionDevice`
 - `cableType`
 - `cableCrossSection`
 - `cableLength`
-- `rcdAssignment` optional
 - `terminalDesignation` optional
-- `voltage` optional
+- `voltage`, derived from project settings and phase category
 - `controlRequirement` optional, for example DALI or KNX
 - `status` optional
 - `isReserve` optional
 - `remark` optional
 
+`protectionDevice` is the circuit-owned one-to-one protection configuration.
+It contains the device type, rated current and only the type-dependent values
+such as trip characteristic or residual current. Newly created circuits
+receive the category default defined in the protection catalog.
+
 Circuit-level fields only:
 
-- protection type
-- protection rated current
-- protection characteristic
+- protection device and all of its technical values
 - cable type
 - cable cross-section
 - cable length
-- RCD assignment
 - terminal designation
 - circuit total power