fix(date_range): snap weekly start to anchor before generating dates

For W/W-MON etc. frequencies, genFromStart and genBetween now call
snapToAnchor() to advance the cursor to the first occurrence of the
anchor weekday on or after the provided start date, matching pandas
behaviour where date_range('2024-01-01', periods=4, freq='W') returns
four Sundays rather than a Monday followed by three Sundays.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: github-actions[bot]

src/stats/date_range.ts

@@ -413,10 +413,24 @@ function retreatDate(d: Date, pf: ParsedFreq): Date {
 
 // ─── generation helpers ────────────────────────────────────────────────────────
 
+/**
+ * For anchor-based frequencies (e.g. "W"), snap `d` forward to the first
+ * occurrence of the anchor day on or after `d`.  For all other frequencies
+ * the date is returned unchanged.
+ */
+function snapToAnchor(d: Date, pf: ParsedFreq): Date {
+  if (pf.unit === "W") {
+    const dow = d.getUTCDay();
+    const daysUntil = (pf.anchor - dow + 7) % 7;
+    return daysUntil === 0 ? d : new Date(d.getTime() + daysUntil * MS_DAY);
+  }
+  return d;
+}
+
 /** Generate `count` dates starting from `start`, advancing by `pf` each step. */
 function genFromStart(start: Date, count: number, pf: ParsedFreq): Date[] {
   const out: Date[] = [];
-  let cur = start;
+  let cur = snapToAnchor(start, pf);
   for (let i = 0; i < count; i++) {
     out.push(cur);
     cur = advanceDate(cur, pf);
@@ -430,7 +444,7 @@ function genFromStart(start: Date, count: number, pf: ParsedFreq): Date[] {
  */
 function genBetween(start: Date, end: Date, pf: ParsedFreq): Date[] {
   const out: Date[] = [];
-  let cur = start;
+  let cur = snapToAnchor(start, pf);
   while (cur.getTime() <= end.getTime()) {
     out.push(cur);
     const next = advanceDate(cur, pf);