Answer the second stability pass: the emitter's own refusal, and the comparator's third arm

AGENTS.md

@@ -172,7 +172,8 @@ both answer to the round-trip corpus and to nothing else where a node has no fix
   directions must answer alike — whether a list marker interrupts a paragraph — is one function
   there too, never a copy per direction, however conservative the copy would be. Where the rule is
   the emitter's own choice, input consults it rather than restating it: the parser asks
-  `spellsCommonMark` which form the emitter picks, so no fixture the emitter writes can be refused.
+  `commonMarkSpelling` which form the emitter picks, so no fixture the emitter writes can be
+  refused, and a spelling the emitter refuses gives its own error rather than a second name for it.
 - The attribute vocabulary is ADF's: `adf/` walks it and narrows each value to its kind, and a
   format spells the narrowed value. A spelling that re-checks the type is the check's second copy.
   Reading a spelling back is the format's own: the reader sits beside the spelling it inverts, so