fix(gate): the plan-code guard catches decision codes, and the purge takes them

A plan's decision ids (`D1`, `D4`, `D6-A`) cited in 11 comments and test titles named a document
no reader of this repo has, and the guard had no pattern for them. It has now (mirrored in
audit_metrics), with cases for each form the citations took and for the look-alikes it must leave
alone (a JPEG marker, "3D"); the 11 lines say their why without the id. Case test seen red without
the pattern, the scan red on a planted `D9:`.

frontend/src/components/UpNextCard.tsx

@@ -10,7 +10,7 @@ import { Button } from "./ui/Button";
 import { IconButton } from "./ui/IconButton";
 
 // Up-next / end-card. Shown at episode end when a next item exists and the pref is on, INSTEAD
-// of the old instant jump — so binge stays under the viewer's control (D6-A). Counts down ~10s then
+// of the old instant jump — so binge stays under the viewer's control. Counts down ~10s then
 // advances; "Play now" advances immediately; Cancel/✕ dismisses and stays on the finished frame. It only
 // lives on the play-to-the-end path; credits-auto-skip advances earlier and never reaches this card.
 const COUNTDOWN_S = 10;