feat: semantic (embedding-based) response cache

- cache.rs: cosine-similarity store (threshold, TTL, ring buffer), partitioned by
  routing signature; tracks hit rate and model cost saved
- pipeline: embed routing text, look up before the model call (route_source=cache on
  hit), insert successful responses; stats record cache hits
- config [cache] + admin settings card; dashboard shows hit-rate and savings
- embedding model resolved from the pool (or first kind="embedding")

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -19,6 +19,10 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/).
   SSE), so Claude-native clients can target the gateway.
 - **Embeddings**: `POST /v1/embeddings` inbound + provider `embed()`; embedding models
   are now resolvable from the pool.
+- **Semantic cache**: embedding-based cache that returns a stored answer for prompts
+  semantically near a previous one (cosine threshold, TTL, ring buffer), skipping the
+  model call. Partitioned by routing signature; hit-rate and cost saved are tracked and
+  shown on the dashboard; configurable from Settings (`[cache]`).
 
 ## [0.1.0] - 2026-06-30