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/0007_watery_kingpin.sql

@@ -0,0 +1,14 @@
+CREATE TABLE `cable_sizing_calculations` (
+	`id` text PRIMARY KEY NOT NULL,
+	`project_id` text,
+	`circuit_id` text,
+	`equipment_identifier` text,
+	`input` text NOT NULL,
+	`result` text NOT NULL,
+	`applied_to_circuit` integer DEFAULT 0 NOT NULL,
+	`created_at` integer DEFAULT (unixepoch()) NOT NULL,
+	FOREIGN KEY (`project_id`) REFERENCES `projects`(`id`) ON UPDATE no action ON DELETE set null,
+	FOREIGN KEY (`circuit_id`) REFERENCES `circuits`(`id`) ON UPDATE no action ON DELETE set null
+);
+--> statement-breakpoint
+CREATE INDEX `cable_sizing_calculations_circuit_id_idx` ON `cable_sizing_calculations` (`circuit_id`);