[Feature] Add process-local in-memory KV indexer and Router integration (#33370)

Co-authored-by: Wu, Yutong <yutong.wu@amd.com>
Co-authored-by: TianDi101 <ditian12@amd.com>
Co-authored-by: Zhangheng <hzh0425@apache.org>

experimental/sgl-router/README.md

@@ -50,6 +50,26 @@ Omit `--service-discovery-namespace` to watch all namespaces (requires
 cluster-wide RBAC). For prefill/decode disaggregation, replace `--selector`
 with `--prefill-selector` and `--decode-selector`.
 
+External KV indexer as the cache-aware signal source:
+
+```bash
+sgl-router \
+  --model-id qwen3 \
+  --tokenizer-path /models/qwen3/tokenizer.json \
+  --worker-urls http://10.0.0.1:30000 http://10.0.0.2:30000 \
+  --policy cache_aware_zmq \
+  --kv-indexer-endpoint http://10.0.0.10:50051 \
+  --kv-indexer-query-timeout-ms 100 \
+  --kv-indexer-query-max-inflight 32
+```
+
+The existing cache-aware policy and thresholds are reused. When configured, the
+Indexer replaces the Router-local radix tree as the cache signal. A successful
+query with no usable match selects by minimum active load; connection failures,
+timeouts, local admission rejection, and server rejection fail the Router
+request with `503` rather than silently switching signals. The timeout and local
+concurrency bound default to 100ms and 32 respectively.
+
 ## License
 
 Apache-2.0.