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/spec/08-current-product-backlog.md

@@ -46,7 +46,9 @@ requirements and intended sequencing, not proof of implementation.
 - [x] Phase E: editor projection and editing.
 - [x] Phase F: documentation and full GUI verification.
 - [ ] Keep full electrical sizing and cable-dimensioning rules separate until
-  the planner supplies the calculation requirements.
+  the planner supplies the calculation requirements. The optional per-circuit
+  cable sizing module (`docs/cable-sizing-module.md`) exists; its tables must
+  be confirmed by the planner against DIN VDE 0298-4.
 - Detailed specification:
   `docs/spec/09-distribution-board-components-and-protection-groups.md`