Part the inline scan into named readers, and share the mark identity both directions read

todo.md

@@ -308,7 +308,9 @@ detail is settled at its own milestone.
         canonical fixpoint: a named error, or markdown that parses and emits to itself byte for
         byte. That HTML's text, tags stripped and entities decoded, against the parsed document's
         concatenated `text`. And a count of the dozen elements the CommonMark subset covers
-        against the marks and nodes they map to. The fixpoint alone is self-consistency a parser
+        against the marks and nodes they map to — counting distinct mark types per text node, since
+        3e collapses a spelling nested inside its own kind and `*(*a*)*` is two `<em>` against one
+        `em`. The fixpoint alone is self-consistency a parser
         returning the empty document passes, and the text alone one dropping every emphasis; the
         counts close both. The exception list stays the maintainer's. One outcome is no
         exception and must not be filed as one: valid CommonMark parsing to a document