Sync testdata with DateKeys spec-v0.8.2 (datekeys-go 9ac9cd9)

The final commit of spec v0.8.2, tagged spec-v0.8.2 after the author's
approval. Only testdata/README.md changes: it says that the mutation
corpus registers its extensions in both arrays of every object, like a
Go Registry without extension.Placement. No fixture or vector changes,
and no Go error text of steps 1 to 8 changes: the second-round
corrections touch step 9 and the drand client, which App does not
implement yet. npm run verify is green: 2,379 tests.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

testdata/README.md

@@ -231,8 +231,14 @@ reading flow (`capsule.Open`, §63) must fail.
   `profile_quicknet.json`; `empty` pins none.
 - `extensions`: the extensions the application implements. Each entry is known
   at `(id, version)`, and its data is valid only when it equals the bytes of
-  `valid_data`. Absent: the application knows no extension, the state of the
-  base protocol V1.
+  `valid_data`. Each entry is registered in both arrays of every object
+  (§72), as a Go `extension.Registry` that does not implement
+  `extension.Placement`: an extension outside its registration counts as
+  unknown there (§54), so the cases "known critical PUBLIC_HEADER extension
+  with invalid data", "known critical CONTROL_CBOR extension with invalid
+  data" and "known critical .dkk extension with invalid data", at steps 4, 14
+  and 9, depend on it. Absent: the application knows no extension, the state
+  of the base protocol V1.
 - `network`: whether the failure may come after a release request. When false,
   the reader must fail without requesting any release (§27, §63): every failure
   of steps 1 to 8 and of step 9 before the request (9.a to 9.c).

testdata/SOURCE.json

@@ -1,8 +1,8 @@
 {
   "module": "g.activething.com/go/DateKeys",
-  "commit": "c57ed4869c94746b352ed4208d06d9df40d6c97e",
+  "commit": "9ac9cd952f048a8c034d59a52b3c7baeffea6ade",
   "files": {
-    "README.md": "25d54741056cf5b4869f4d484b4e292ca48b14bed39a3f063d8014160bcd866c",
+    "README.md": "23d7cd2914c2f2d30aad49ce09119a7dd65383bc0011d494dbad8cfe724f68c5",
     "fixtures/empty_payload.dkc": "871e9bf05b52bbae17f3adfbbf97b46e7f0e53aa8f57bcaa506e43f36f53a9d4",
     "fixtures/empty_payload.inspect.json": "dd2b21faefdfe3aa59b5eeef9130a6896070ae30a2b16933f3aefd146828207e",
     "fixtures/empty_payload.json": "490a6e7f7acc882bcb956b297ad8c817d7b21da29e8d4c4a2cd31c852707eee0",