The preceding commit added lru.Cache for the compiled-regexp cache to
match the style in pkg/sqlite/regex.go. That file's use case is
different: a small bounded cache serving a read-dominated workload. The
auto-tag regexp cache is job-scoped (so eviction buys us nothing) and
hit by every worker on every candidate (so the LRU's per-Get mutex
becomes contention, measurable under the parallel worker pool).
sync.Map's read-optimised path avoids the contention without changing
any observable behavior. Kept as a separate commit so it can be
reverted independently if upstream prefers the LRU approach — the
first commit stands on its own either way.
93a05fce8f