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/db/migrations/meta/_journal.json

@@ -50,6 +50,13 @@
       "when": 1786043080323,
       "tag": "0006_damp_skrulls",
       "breakpoints": true
+    },
+    {
+      "idx": 7,
+      "version": "6",
+      "when": 1786114831610,
+      "tag": "0007_watery_kingpin",
+      "breakpoints": true
     }
   ]
 }