perf(memory-core): memoize the normalized recall haystack per corpus revision

selectRecallCandidates composed `description\nbody` and let matchScore normalize it
again for every document and every query, ~20ms per query pass at a 4.2MB
corpus. The haystack is now normalized once per document object - which
RecallCorpusCache keeps stable while HEAD holds - and scored through the new
matchScoreNormalized entry point. matchScore keeps its raw-text signature and
delegates, so every existing caller and score is unchanged.

Refs #8335