refactor: improve assign naming and clarify memory usage test helper

Agent-Logs-Url: https://github.com/githubnext/tsessebe/sessions/78d95042-4c45-4c08-be04-02e8afe42b80

Co-authored-by: mrjf <180956+mrjf@users.noreply.github.com>

Author: Copilot

src/core/frame.ts

@@ -310,19 +310,19 @@ export class DataFrame {
       >
     >,
   ): DataFrame {
-    let working: DataFrame = this;
+    let currentFrame: DataFrame = this;
     for (const [name, spec] of Object.entries(newCols)) {
       const resolved: readonly Scalar[] | Series<Scalar> =
-        typeof spec === "function" ? spec(working) : spec;
+        typeof spec === "function" ? spec(currentFrame) : spec;
       const series: Series<Scalar> =
         resolved instanceof Series
           ? resolved
-          : new Series({ data: resolved, index: working.index });
-      const colMap = new Map<string, Series<Scalar>>(working._columns);
+          : new Series({ data: resolved, index: currentFrame.index });
+      const colMap = new Map<string, Series<Scalar>>(currentFrame._columns);
       colMap.set(name, series);
-      working = new DataFrame(colMap, working.index);
+      currentFrame = new DataFrame(colMap, currentFrame.index);
     }
-    return working;
+    return currentFrame;
   }
 
   /** Drop one or more columns by name.  Returns a new DataFrame. */

tests/stats/memory_usage.test.ts

@@ -19,6 +19,7 @@ function dfFromSeries(columns: Record<string, Series<Scalar>>): DataFrame {
   for (const [name, series] of Object.entries(columns)) {
     colMap.set(name, series);
   }
+  // Build through the low-level constructor to preserve explicit Series dtypes in tests.
   const firstCol = Object.values(columns)[0];
   const index = new RangeIndex(firstCol?.size ?? 0);
   return new DataFrame(colMap, index);