refactor: decouple table loading from lookup generation

Extract `loadReferenceTableData` to separate the raw file loading
logic from the primary key lookup construction. This prevents
unnecessary Map creation during reverse reference resolution.

src/csv-loader/reference-resolver.ts

@@ -41,13 +41,11 @@ export function hasNestedReferences(schema: Schema): boolean {
   }
 }
 
-export function loadReferenceTable(
+function loadReferenceTableData(
   schema: ReferenceSchema | ReverseReferenceSchema,
   refBaseDir: string | undefined,
-  defaultPrimaryKey: string,
   currentFilePath: string | undefined,
 ): {
-  lookup: Map<string, Record<string, unknown>>;
   refTable: Record<string, unknown>[];
   refFilePath: string;
 } {
@@ -86,6 +84,30 @@ export function loadReferenceTable(
     }
   }
 
+  return { refTable, refFilePath };
+}
+
+/**
+ * Load a referenced table and build a forward lookup keyed by primary key.
+ * Only call this when forward resolution is actually needed; the reverse
+ * path uses `loadReferenceTableData` directly to avoid building a throwaway map.
+ */
+export function loadReferenceTable(
+  schema: ReferenceSchema | ReverseReferenceSchema,
+  refBaseDir: string | undefined,
+  defaultPrimaryKey: string,
+  currentFilePath: string | undefined,
+): {
+  lookup: Map<string, Record<string, unknown>>;
+  refTable: Record<string, unknown>[];
+  refFilePath: string;
+} {
+  const { refTable, refFilePath } = loadReferenceTableData(
+    schema,
+    refBaseDir,
+    currentFilePath,
+  );
+
   const lookup = new Map<string, Record<string, unknown>>();
   refTable.forEach((row) => {
     const pkValue = row[defaultPrimaryKey];
@@ -107,10 +129,9 @@ function getReverseLookup(
   defaultPrimaryKey: string,
   currentFilePath: string | undefined,
 ): Map<string, Record<string, unknown>[]> {
-  const { refTable, refFilePath } = loadReferenceTable(
+  const { refTable, refFilePath } = loadReferenceTableData(
     schema,
     refBaseDir,
-    defaultPrimaryKey,
     currentFilePath,
   );