feat(cache): add SemanticCache component backed by RagProviderInterface

[usernane]

Summary

Add a SemanticCache component that matches chat requests by vector similarity, backed by an existing RagProviderInterface (ADR-0045) for storage/retrieval and a ProviderInterface for query embeddings. See ADR-0049.

Design

SemanticCache is a standalone class, not a CacheInterface implementation, because the lookup model differs fundamentally (text similarity vs. string-key equality). See ADR-0049 "Alternatives Considered".

class SemanticCache {
    public function __construct(
        RagProviderInterface $store,
        ProviderInterface $embedder,
    ) {}

    public function lookup(string $query, SemanticCacheConfig $config): ?ChatResponse {}
    public function store(string $query, ChatResponse $response, SemanticCacheConfig $config): void {}
    public function clear(): void {}
}

lookup()

1. Call $store->retrieve($query, topK: 1).
2. If no result or result.score < $config->getThreshold() → return null (miss).
3. Reconstruct ChatResponse from result.getMetadata()['cached_response'] via ChatResponse::fromArray().

store()

1. Serialize the response via ChatResponse::toArray().
2. $store->ingest($query, ['cached_response' => $arr, 'model' => ..., 'provider' => ..., 'created_at' => time()]).

Dependencies

• Blocked by feat(cache): add toArray/fromArray serialization to ChatResponse object graph #143 (ChatResponse serialization) — lookup()/store() need toArray()/fromArray().
• Uses RagProviderInterface + RetrievalResult (already provide score + freeform metadata, verified sufficient — no RAG interface changes needed).

Acceptance Criteria

• SemanticCache class with lookup(), store(), clear()
• Threshold check uses RetrievalResult::getScore()
• Response reconstructed from metadata
• TTL respected (via config; store-level or metadata timestamp check)
• Unit tests with a fake RagProviderInterface covering: hit above threshold, miss below threshold, empty store, round-trip of stored response
• composer test green

References

• ADR-0049: AI: Semantic Response Caching
• ADR-0045: RagProviderInterface