The attribute-value scanner ran the entity decoder on every value, and the
decoder re-scanned the whole value for '&' before doing nothing when none
was present. The byte-run scanner already visits every value byte, so it
now notes any '&' and an ampersand-free value (the common case) returns its
normalized run directly. Measured against a same-machine-state baseline,
every run came in below every baseline run, ~4% faster parse on the
20k-item generated corpus.
Move the bulk byte-run scanners (contentRunBytes, attributeRunBytes) into
ReaderByteRuns.swift and centralize their cursor mutation through a single
advanceByteRun method, so Reader's position fields stay write-encapsulated
in Reader.swift while both files stay under the length cap. The byte and
Character paths are held to identical decoded values by the differential
oracle, extended with ampersand-free, entity, and character-reference
attribute cases. No spec behavior changed.
