feat: resolve unit price from catalog with selection override

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

mvp/b2c/src/budget/resolve.ts

@@ -0,0 +1,18 @@
+import type { Calidad, CategoriaMaterial, CatalogItem } from './types';
+
+export function resolvePrecioUnitario(
+  categoria: CategoriaMaterial,
+  calidad: Calidad,
+  catalog: CatalogItem[],
+  selections: Partial<Record<CategoriaMaterial, string>>,
+): { item: CatalogItem | null } {
+  const selectedId = selections[categoria];
+  if (selectedId) {
+    const selected = catalog.find((c) => c.id === selectedId);
+    if (selected) return { item: selected };
+  }
+  const def = catalog.find(
+    (c) => c.categoria === categoria && c.calidad === calidad && c.esDefault,
+  );
+  return { item: def ?? null };
+}