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>

AGENTS.md

@@ -507,9 +507,31 @@ Before implementation, audit the current ProjectDevice, CircuitDeviceRow,
 command, snapshot, transfer and editor drag-and-drop paths. Propose concrete
 domain objects and commands before adding migrations.
 
+## Cable Sizing Module
+
+`src/cable-sizing` is an agreed, separate calculator module attached to the
+circuit list (see `docs/cable-sizing-module.md`). Rules:
+
+- The domain code stays pure: no imports from `db/`, `server/` or `frontend/`.
+- It never writes project data. Results are applied only by an explicit
+  "Übernehmen", which runs the regular `circuit.update` command.
+- It is opened from the circuit row action button "Leitungsberechnung", never
+  by clicking a grid cell; cable cells keep the normal spreadsheet behavior.
+- It has no database table and no migration; `/api/cable-sizing/*` is
+  stateless.
+- Reference data is IEC 60364-5-52:2009 Annex B. Change table values only with
+  a documented source in `docs/cable-sizing-module.md`; where sources differ,
+  use the lower value. Inputs outside the tables return no result instead of
+  assuming a factor of 1.
+- Inputs are prefilled from the circuit (power, phase, voltage, weighted cos φ,
+  length, protective device) and remain editable; manual cable entry without
+  a calculation stays possible.
+
 ## Future Sizing
 
-Do not implement full cable/protection sizing unless explicitly requested.
+Do not implement full automatic cable/protection sizing of all circuits
+unless explicitly requested. The cable sizing module above only suggests
+values for one circuit on demand.
 
 Keep the structure ready for it.