feat(freight): offer built-train wagons per boarding yard on multi-yard consists

apps/edr-freight-api/src/modules/train-scheduling/wagon-plan-flex.util.ts

@@ -43,6 +43,16 @@ export type WagonStock = {
   remainingByTypeId: Map<string, number>;
   /** Wagon-type code per id, for human-readable shortfall messages. */
   codesByTypeId: Map<string, string>;
+  /**
+   * Multi-yard consist only: yardId → (wagonTypeId → count) for the wagons
+   * standing at that yard. A train whose wagons are split across yards can
+   * only offer, at each boarding yard, the wagons physically standing there —
+   * a wagon waiting in Mojo is not bookable from Dire, and one picked up at
+   * Dire is not re-offered at Mojo. Absent (undefined) when every wagon sits
+   * in one yard, which keeps single-yard trains on the original whole-train
+   * math.
+   */
+  byYardId?: Map<string, Map<string, number>>;
 };
 
 export type FlexPlanResult = {