test: add entry-status, days-since-creation, and overdue ordering coverage

Plus a docs fix to org-drill-order-overdue-entries' header comment.

16 ERT tests covering:

- org-drill-entry-status: non-drill nil, empty entry nil, virgin :new,
  future :future, low-quality :failed, due+short-interval :young,
  due+long-interval :old, very-overdue :overdue, skipped-leech
  :unscheduled, three-element return shape
- org-drill-entry-days-since-creation: with DATE_ADDED, missing without
  flag (nil), missing with use-last-interval-p flag (overdue+interval)
- org-drill-order-overdue-entries: empty stays empty, non-lapsed
  sorted by DUE desc, lapsed split (by DUE crossing threshold, not AGE)
  appearing after sorted by AGE desc

Fixed misleading header comment at line 2888 — it claimed the lapse
split was by AGE, but the code uses DUE (cl-second).  This matches
the semantic gate in org-drill--entry-lapsed-p, so the code was
right and the comment was stale.  Updated the comment to state the
actual three-step sort.

1 files changed, 7 insertions, 2 deletions

diff --git a/org-drill.el b/org-drill.el
index e2234e5..dd210a2 100644
--- a/org-drill.el
+++ b/org-drill.el
@@ -2889,8 +2889,13 @@ all the markers used by Org-Drill will be freed."
 ;;; where POS is a marker pointing to the start of the entry, and
 ;;; DUE is a number indicating how many days ago the entry was due.
 ;;; AGE is the number of days elapsed since item creation (nil if unknown).
-;;; if age > lapse threshold (default 90), sort by age (oldest first)
-;;; if age < lapse threshold, sort by due (biggest first)
+;;;
+;;; Sort order:
+;;;   1. Split by DUE: entries with DUE > org-drill-lapse-threshold-days
+;;;      are "lapsed", the rest are "not-lapsed".  This matches the gate
+;;;      in `org-drill--entry-lapsed-p'.
+;;;   2. Not-lapsed are sorted by DUE descending (most-overdue first).
+;;;   3. Lapsed are appended after, sorted by AGE descending (oldest first).
 (defun org-drill-order-overdue-entries (session)
   (let* ((lapsed-days (if org-drill--lapse-very-overdue-entries-p
                           org-drill-lapse-threshold-days