firstLineOf (query) and truncateString (edit) sliced strings at a raw byte
offset, splitting a multibyte UTF-8 character into invalid bytes in
LLM-facing output (ast_query compact format and edit not-found errors are
hot paths). Extract the rune-boundary backoff already used by search's
truncateLine into util.RuneBoundary and route all three sites through it.
Adds table-driven tests for the primitive and a multibyte case for firstLineOf.
