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.

tests/cable-sizing-calculation.test.ts

@@ -0,0 +1,126 @@
+import assert from "node:assert/strict";
+import { describe, it } from "node:test";
+import {
+  buildCableSizingAlerts,
+  calculateCableSizing,
+  isCableSizingDataVerified,
+  type CableSizingInput,
+} from "../src/cable-sizing/domain/cable-sizing-calculation.js";
+
+const BASE_INPUT: CableSizingInput = {
+  phase: 1,
+  mode: "power",
+  powerKw: 5,
+  cosPhi: 1,
+  voltage: 230,
+  lengthM: 30,
+  layingMethod: "C",
+  conductorMaterial: "copper",
+  insulation: "pvc",
+  ambientTemperatureC: 30,
+  groupingCircuits: 1,
+  maxVoltageDropPercent: 3,
+  harmonicNeutralLoad: "none",
+};
+
+describe("calculateCableSizing", () => {
+  it("matches the verified reference case (1~, 5 kW, 230 V, 30 m, method C, copper)", () => {
+    const result = calculateCableSizing(BASE_INPUT);
+    assert.equal(result.dataVerified, true);
+    assert.ok(Math.abs(result.operatingCurrentA - 21.739) < 0.01);
+    assert.equal(result.crossSectionByCapacityMm2, 2.5);
+    assert.equal(result.crossSectionByVoltageDropMm2, 4);
+    assert.equal(result.recommendedCrossSectionMm2, 4);
+  });
+
+  it("returns dataVerified: false and no numeric result for an unverified laying method", () => {
+    const result = calculateCableSizing({ ...BASE_INPUT, layingMethod: "A2" });
+    assert.equal(result.dataVerified, false);
+    assert.equal(result.recommendedCrossSectionMm2, null);
+    assert.equal(result.rows.length, 0);
+    // The operating current itself does not depend on the capacity table
+    // and is still reported so the UI can show at least that much.
+    assert.ok(Math.abs(result.operatingCurrentA - 21.739) < 0.01);
+  });
+
+  it("returns dataVerified: false for xlpe regardless of method", () => {
+    const result = calculateCableSizing({ ...BASE_INPUT, insulation: "xlpe" });
+    assert.equal(result.dataVerified, false);
+  });
+
+  it("isCableSizingDataVerified matches the six ported, verified methods only", () => {
+    for (const method of ["A1", "B2", "C", "E", "D1", "D2"] as const) {
+      assert.equal(isCableSizingDataVerified(method, "pvc"), true, method);
+    }
+    for (const method of ["A2", "B1", "F", "G"] as const) {
+      assert.equal(isCableSizingDataVerified(method, "pvc"), false, method);
+    }
+  });
+
+  it("applies the 0.86 harmonic reduction factor only for three-phase + 15to33Percent", () => {
+    const threePhase: CableSizingInput = {
+      ...BASE_INPUT,
+      phase: 3,
+      mode: "current",
+      currentA: 10,
+      powerKw: undefined,
+      voltage: 400,
+    };
+    const base = calculateCableSizing({ ...threePhase, harmonicNeutralLoad: "none" });
+    const derated = calculateCableSizing({
+      ...threePhase,
+      harmonicNeutralLoad: "15to33Percent",
+    });
+    assert.equal(base.harmonicReductionApplied, false);
+    assert.equal(derated.harmonicReductionApplied, true);
+    assert.ok(
+      Math.abs(derated.combinedDerationFactor - base.combinedDerationFactor * 0.86) < 1e-9
+    );
+
+    const singlePhaseWithHarmonics = calculateCableSizing({
+      ...BASE_INPUT,
+      harmonicNeutralLoad: "15to33Percent",
+    });
+    assert.equal(singlePhaseWithHarmonics.harmonicReductionApplied, false);
+  });
+
+  it("flags a simplified protection coordination mismatch", () => {
+    const result = calculateCableSizing({
+      ...BASE_INPUT,
+      existingProtectionRatedCurrentA: 1000,
+    });
+    assert.ok(result.protectionCoordination);
+    assert.equal(result.protectionCoordination?.coordinated, false);
+
+    const alerts = buildCableSizingAlerts(
+      { ...BASE_INPUT, existingProtectionRatedCurrentA: 1000 },
+      result
+    );
+    assert.ok(alerts.some((alert) => alert.text.includes("Koordination prüfen")));
+  });
+});
+
+describe("buildCableSizingAlerts", () => {
+  it("returns a single critical alert for an unverified combination, no numeric claims", () => {
+    const input: CableSizingInput = { ...BASE_INPUT, layingMethod: "G" };
+    const result = calculateCableSizing(input);
+    const alerts = buildCableSizingAlerts(input, result);
+    assert.equal(alerts.length, 1);
+    assert.equal(alerts[0].kind, "critical");
+    assert.ok(alerts[0].text.includes("keine geprüften"));
+  });
+
+  it("returns a single ok alert for a clean, unremarkable case", () => {
+    const input: CableSizingInput = {
+      ...BASE_INPUT,
+      mode: "current",
+      currentA: 15,
+      powerKw: undefined,
+      lengthM: 3,
+    };
+    const result = calculateCableSizing(input);
+    const alerts = buildCableSizingAlerts(input, result);
+    assert.equal(alerts.length, 1);
+    assert.equal(alerts[0].kind, "ok");
+  });
+});