vector: write_admitted messageId refuses U+FEFF before a letter (kills a trim-before-check decoder)

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

contracts/cultnet/interop/write-receipt-vectors.json

@@ -351,6 +351,15 @@
         "expect": "refuse",
         "why": "U+2060 is Default_Ignorable"
       },
+      {
+        "name": "refuse_bom_before_letter",
+        "codePoints": [
+          "FEFF",
+          "0061"
+        ],
+        "expect": "refuse",
+        "why": "U+FEFF is Default_Ignorable; JavaScript trim() would strip it and accept the rest, but the compare is raw"
+      },
       {
         "name": "accept_abc",
         "codePoints": [
@@ -1467,7 +1476,7 @@
     "schemaInvalid": 7,
     "contentHashCases": 4,
     "scenarioCases": 25,
-    "admittedMessageIdCases": 33
+    "admittedMessageIdCases": 34
   },
   "contentHash": {
     "definition": "sha256(msgpack([operation, schemaId, recordKey, payload])), payload a msgpack bin for a put and nil for a delete; hex lowercase",