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/PlexBrowse.tsx

@@ -68,7 +68,7 @@ const PlexCollectionView = lazy(() => import("./PlexCollectionView"));
 
 const PAGE = 40;
 
-// D3: the "last watched" sort auto-applied when you enter the in-progress watch-state view (most
+// The "last watched" sort auto-applied when you enter the in-progress watch-state view (most
 // recently watched first). Frozen for the same reason as PLEX_SORT_FALLBACK (handed out by reference).
 const LASTWATCHED_SORT: NonNullable<SortState> = Object.freeze({
   key: "lastwatched",
@@ -229,7 +229,7 @@ export default function PlexBrowse() {
     return () => window.removeEventListener("keydown", onKey);
   }, [sub, canStepBack]);
 
-  // D3: entering the in-progress watch-state view auto-applies the "last watched" sort (most recently
+  // Entering the in-progress watch-state view auto-applies the "last watched" sort (most recently
   // watched first); leaving restores the sort that was active before entering. Only the ENTER edge
   // forces the sort, so a manual sort change while in the in-progress view still wins — it's just not
   // carried back out (exit returns to the pre-entry preference). We stash the raw provider `sort`, not