feat: P0 production hardening — LLM cache, semantic routing, state persistence
U1: LLM Cache Core (exact + semantic match, InMemory + Redis backends)
U2: Cache integration into LLMGateway with CacheConfig
U3: Semantic Router as Layer 1.5 in CostAwareRouter
U4: UsageStore persistence (Redis Hash + InMemory fallback)
U5: CascadeStateStore persistence (Redis INCR + InMemory TTL)
U6: EvolutionStore interface unification (Protocol + PostgreSQL backend)
U7: Configuration integration + E2E tests
Code review fixes:
- P0: date iteration bug (day>=28), semantic router index never built,
Redis connection leak (per-call → persistent pool)
- P1: cache degradation recovery, semantic_search degradation,
double miss counting, asyncio.Lock for PG init, LIMIT on queries,
__import__ anti-pattern → _utcnow()
- P2: InMemory TTL cleanup, embedding preservation on put(),
data TTL = max(exact_ttl, semantic_ttl)
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