feat(cag): memory standing-context adapter, warmup semaphore, cache-aware routing

Issue #44: code-register memory adapter with standing-context resource; fix
dispatcher method-level Auth.Capability override (was always using verb-derived
capability, ignoring per-method Auth field).

Issue #45: per-backend bounded in-memory queue (VLLM_MAX_QUEUE_DEPTH=20) +
concurrency semaphore (VLLM_MAX_NUM_SEQS=4); warmup requests (X-Poimen-Warmup:
true) sleep WARMUP_DELAY_MS before queue entry; /metrics endpoint wired.

Issue #46: rendezvous-hash backend selection on X-Poimen-Cache-Key; round-robin
fallback; MarkFailed excludes unhealthy backends 30s; ModifyResponse captures
vllm-num-cached-tokens + emits structured log + increments counter.

Closes #44 Closes #45 Closes #46

internal/proxy/router.go

@@ -159,6 +159,7 @@ func (h *Handler) routeByModel(r *http.Request, path string) (*Route, error) {
 
 	route := &Route{
 		Name:      "v1-chat-" + modelName,
+		ModelName: modelName,
 		Upstream:  upstreamCfg,
 		Transport: h.getOrCreateTransport(modelUpstream.Address, upstreamCfg),
 		Director: func(req *http.Request) {