Adds passive tag-based invalidation modeled on FusionCache: cache
entries carry a set of tags, backends store invalidation timestamps
per tag, and on read the FSM compares per-tag timestamps against the
entry's creation time. No tag→keys index, no eager eviction — entries
are treated as expired when any of their tags has a newer invalidation
timestamp.

FSM integration runs request-side tag invalidation prefetch in
parallel with the cache read, attaches response-side tags at write
time, and short-circuits the parallel fetch when no tags are
configured (zero overhead for non-tag flows).

Includes BDD coverage for invalidation behavior and backend-level tag
round-trip / invalidate / invalidated tests across Moka, FeOxDb, and
Redis.