Add optional cable-sizing module

Per-circuit cross-section recommendation as a separate module attached to
the circuit list. It never writes project data itself: "Übernehmen" applies
cable type, cross-section and length through the regular circuit.update
command, so revisions and Undo/Redo stay intact.

- Pure domain module (src/cable-sizing) with IEC 60364-5-52:2009 Annex B
  tables for PVC and XLPE, copper and aluminium, methods A1, A2, B1, B2, C,
  D1, D2 and E. Every value was cross-checked against public reproductions;
  where sources differ the lower value is used (docs/cable-sizing-module.md).
- Temperature and grouping between table values round to the safer value;
  values outside the tables return no result instead of a factor of 1.
  Ground methods use B.52.18 (D2) and B.52.19 A (D1).
- Overload protection: circuit-breakers In <= Iz; gG fuses additionally
  I2 <= 1.45 Iz with I2 = 1.6/1.9/2.1 In.
- Inputs prefilled from the circuit: power, phase (group or device rows),
  project voltage, power-weighted cos phi, length and protective device.
- Opened from a "Leitungsberechnung" button in the row actions, not from a
  grid cell, so cable cells keep the spreadsheet behavior.
- Stateless API under /api/cable-sizing; no database table or migration.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/circuit-list-editor-api.md

@@ -391,6 +391,28 @@ inverse command. The former direct restore/reconnect endpoints are removed.
 `displayName` is included in the comparison but is not selected by default in the UI. Updating a
 project device never triggers synchronization implicitly.
 
+## Cable Sizing (optional module)
+
+Stateless calculator endpoints mounted at `/api/cable-sizing`. They neither
+read nor write project data; applying a result uses `circuit.update` through
+`POST /projects/:projectId/commands`.
+
+- `POST /cable-sizing/calculate`
+  - request: phase, mode (`power` | `current`), powerKw or currentA, cosPhi,
+    voltage, lengthM, layingMethod (`A1` | `A2` | `B1` | `B2` | `C` | `D1` |
+    `D2` | `E`), conductorMaterial, insulation (`pvc` | `xlpe`),
+    ambientTemperatureC, groupingCircuits, maxVoltageDropPercent,
+    harmonicNeutralLoad, optional `existingProtection`
+    (`{ ratedCurrentA, kind: "breaker" | "fuse" }`) and circuitCategory
+  - returns `{ "result": { ... }, "alerts": [ ... ] }`; `result.supported`
+    is `false` with `unsupportedReason` when temperature or grouping lie
+    outside the tables
+  - unknown fields are rejected (`400`)
+- `GET /cable-sizing/laying-methods`
+- `GET /cable-sizing/insulation-materials`
+
+Full request example and calculation rules: `docs/cable-sizing-module.md`.
+
 ## Revit CSV Foundation
 
 - `GET /projects/:projectId/external-csv/configuration`