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/PlexDiscover.dom.test.tsx

@@ -273,7 +273,7 @@ describe("PlexDiscover", () => {
 });
 
 // Every rail and the genre grid are lists the keyboard walks (lib/listNav), so `j` goes on
-// from one to the next in page order (owner decision D4).
+// from one to the next in page order (the owner's call).
 describe("PlexDiscover · keyboard", () => {
   it("registers every rail and the genre grid, and j walks on from rail to rail", async () => {
     plexDiscover.mockResolvedValue(fullPayload);