docs: state that a root string starting with U+FEFF is quoted

SPEC.md

@@ -424,6 +424,7 @@ Encoders MUST quote a string value if any of the following is true:
 - It contains the relevant delimiter – the active delimiter for inline array values, tabular row cells, and keyed entry-row cells; the document delimiter for object field values (§11.1, which is authoritative for delimiter-aware quoting).
 - It equals "-" or starts with "-" (any hyphen at position 0).
 - It equals "#" or starts with "#" (any number sign at position 0).
+- It is a root primitive (§5) and starts with U+FEFF (§12).
 
 Otherwise, the string MAY be emitted without quotes. Unicode, emoji, and strings with internal (non-leading/trailing) spaces are safe unquoted provided they do not violate the conditions.
 

tests/fixtures/encode/primitives.json

@@ -273,6 +273,14 @@
       "input": "#hello",
       "expected": "\"#hello\"",
       "specSection": "7.2"
+    },
+    {
+      "name": "quotes a root string starting with U+FEFF",
+      "input": "\ufeff8",
+      "expected": "\"\ufeff8\"",
+      "specSection": "7.2",
+      "note": "Unquoted, a decoder would remove the U+FEFF as a byte-order mark (§12) and read the number 8",
+      "minSpecVersion": "4.1"
     }
   ]
 }