Precise KV-index routing: vLLM KV cache events through the agent, routed by leading blocks held

Plan §7.5 tier 2, opt-in per profile with kv_events. The agent starts vLLM
publishing KV cache events to a loopback port it listens on (pure-Go zmq4,
msgpack in both of vLLM's encodings), with sha256_cbor hashing and a fixed
PYTHONHASHSEED; it mirrors each engine's blocks and sends net changes up the
existing stream, with a full copy on every new one and a reset on gaps,
restarts and clears. The control plane hands them to the router, for the
sending node's own replicas only. The router tokenizes through a replica's
/tokenize, hashes the tokens as vLLM does (checked against cbor2/hashlib
golden values), and scores replicas by the share of leading blocks held,
falling back to the approximate tree. New metrics beside
llmgw_prefix_decisions_total. Untested against a running vLLM.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/admin-api.openapi.json

@@ -5253,6 +5253,10 @@
             "type": "integer",
             "format": "int64",
             "description": "StartupTimeout bounds model loading. A 70B model across two nodes takes tens of minutes, so this is per profile rather than global.\n\nA duration, in nanoseconds."
+          },
+          "kv_events": {
+            "type": "boolean",
+            "description": "KVEvents has vLLM publish what its prefix cache holds, and the router route this model's requests by it: to the replica holding the most of a prompt's leading blocks (plan §7.5 tier 2). vLLM only.\n\nOff by default, because it costs something on every request: the router asks a replica to tokenize the prompt before placing it, a round trip and the chat template run twice. It pays where prompts share long prefixes that the approximate tree guesses wrong — many replicas, conversations that move between them, evictions. The engine is started with sha256_cbor block hashing and a fixed PYTHONHASHSEED, which the router's hashing assumes; arguments overriding either leave the index empty, and the agent says so in its log."
           }
         }
       },