OptionalcreateToken ids that legitimately END an argument value in this template — the
closing quote (JSON) or the markup that follows a value (</parameter).
Read off the diff between two value probes: the first non-whitespace
token their renderings share AFTER the value is what terminates it. The
echo tier uses these to decide where a copied span may stop (and, with a
clean stop, may be asserted rather than verified).
A fresh parser context for each request; compiled rows may be cached.