Add proposed cable-sizing module

Isolated module (mirrors src/external-model/ dependency direction) that
adds an on-demand, DIN VDE 0298-4 referenced cable cross-section
calculator: a click-on-a-circuit input mask for laying method,
insulation, ambient temperature, grouping and voltage-drop limit that
suggests a cross-section for the circuit cableCrossSection/cableLength
fields. Applying a suggestion goes through the existing circuit.update
command (expectedRevision, undo/redo) unchanged - nothing here writes
to circuits directly or touches the revision/command system. See
docs/cable-sizing-module.md for the full rationale, API contract,
verification status and maintenance plan. Proposal, not yet reviewed
by the project owner - see the docs file.

423 existing + 11 new tests pass, build:api/build:web/typecheck:scripts
clean.

src/server/routes/cable-sizing.routes.ts

@@ -0,0 +1,19 @@
+import express from "express";
+import * as cableSizingController from "../controllers/cable-sizing.controller.js";
+
+export const cableSizingRouter = express.Router();
+
+cableSizingRouter.post("/calculate", cableSizingController.calculateCableSizingHandler);
+cableSizingRouter.get("/laying-methods", cableSizingController.listLayingMethodsHandler);
+cableSizingRouter.get(
+  "/insulation-materials",
+  cableSizingController.listInsulationMaterialsHandler
+);
+cableSizingRouter.post(
+  "/calculations/:calculationId/applied",
+  cableSizingController.markCalculationAppliedHandler
+);
+cableSizingRouter.get(
+  "/circuits/:circuitId/calculations",
+  cableSizingController.listCalculationsForCircuitHandler
+);