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/lib/planCodes.contract.test.ts

@@ -24,10 +24,10 @@ const ALLOWED = new Set([
 // Anything else that looks like a code is one — `"""E30:`, `it("E36:`, `# S6:` and `S3's` included.
 const NOT_CODES = /\bS\d{1,2} ?·? ?E\d+|\btitle[=:] ?"[ES]\d{1,2}"|\bE4\/E7\/E9\b/g;
 const PLAN_CODES = [
-  // an epic (`E28`, `E9.5`, `E28/E30`), an older roadmap item (`R14`), a sprint (`S6`, `S3e2`) or a
-  // phase (`P1`) — not `E2E`, not a ruff rule (`S110`), not `S3h`, the media-card decision
-  // docs/DESIGN.md is headed by
-  /\b(?:E\d{1,2}(?:\.\d+)?|R\d{1,2}|S(?!3h\b)\d{1,2}(?:[a-z]\d?)?|P\d)\b/,
+  // an epic (`E28`, `E9.5`, `E28/E30`), an older roadmap item (`R14`), a sprint (`S6`, `S3e2`), a
+  // phase (`P1`) or a plan's decision (`D1`, `D6-A`) — not `E2E`, not a ruff rule (`S110`), not
+  // `S3h`, the media-card decision docs/DESIGN.md is headed by
+  /\b(?:E\d{1,2}(?:\.\d+)?|R\d{1,2}|S(?!3h\b)\d{1,2}(?:[a-z]\d?)?|P\d|D\d{1,2})\b/,
   // a review round (`round 4`, `deep-review`)
   /\b[Rr]ound[ -]\d|deep-review/,
 ];
@@ -58,6 +58,9 @@ describe("plan codes", () => {
       '"""E30: the new sink.',
       'it("E36: keeps x", () => {',
       "// both sides (E28/E30)",
+      "# D1: a newly shared download is downloadable by default",
+      "// binge stays under the viewer's control (D6-A)",
+      "// (owner decision D4)",
       "# the roadmap's R6/R7 pair",
       "// held since E9.5",
       "// (S6) moved it",
@@ -79,6 +82,8 @@ describe("plan codes", () => {
       'expect(btn.textContent).toContain("S2 · E1");',
       "// the S3h media card",
       '"S110", "S112",',
+      "# the JPEG SOI marker (0xFF 0xD8 0xFF)",
+      "// a 3D view",
     ];
     expect(cited.filter((l) => !isPlanCode(l))).toEqual([]);
     expect(clean.filter(isPlanCode)).toEqual([]);