feat: live 2-process disaggregated serving handoff (B3b2a)

[inureyes]

Summary

B3b2a of the disaggregated serving productionization (epic #116, capstone #126). Wires the serving role end to end so two real mlxcel-server processes hand a sequence off over TCP and reproduce the single-node output byte-for-byte. The dedicated router HTTP front and 3-process client E2E land next in B3b2b.

What this does

• Wire protocol (serving_protocol.rs): PrefillRequestFrame (client to prefill), DecodeMetaFrame (prefill to decode, precedes the KV frame), ResultFrame with FirstToken/Continuation phase (nodes to client). Carried as TransportMessage::Control + serde_json.
• Networked role loops (coordinator.rs): run_{prefill,decode}_role_networked read frames off the transport, drive the existing B2b scheduler entries, and return results to each request's reply_to; serve_{prefill,decode}_role_networked_blocking drivers build the runtime and bind the transport.
• Config chain: new --serving-bind <addr> flag on both binaries; serving_bind + decode_peers threaded from ServerStartupInput through ServerConfig and the provider ladder to WorkerSchedulerConfig. prefill_peers stays in ServerConfig (pub) for the future dedicated router.
• Worker LIVE flip (model_worker.rs): a non-hybrid worker binds its role transport and runs the role loop instead of scheduler.run(); a misconfigured role (no --serving-bind, or prefill with no --decode-peers) logs and falls back to the single-node loop rather than hanging.

Design notes

• Result stays split (prefill first token + decode continuation to the client), matching the StreamBridge.submit_first_token / submit_decode_token contract the B3b2b router will merge.
• New --serving-bind: no existing flag gives a node its own role-transport listener (HTTP host:port feeds the cluster-control address).
• Decode intake is two frames (decode-meta control frame then KV frame), keeping the KV serde focused on KV.
• Additive only: the in-crate channel-driven role loops (B3a) and the single-node path are unchanged.

Verification (orchestrator-local, the real metal gate)

• Cold clippy -D warnings both feature sets (metal,accelerate and +test-utils) --all-targets: clean
• cargo fmt --check: clean
• Protocol unit tests 5/5; in-crate B3a/B2c/B3b1 serving parity 4/4 unchanged
• 2-process spawn E2E (tests/disaggregated_serving_e2e.rs): two real mlxcel-server processes hand off over TCP, merged output " \n\n###Answer: 4\n\n###Step 1: 2 + " is byte-identical to the single-node greedy reference
• Full lib suite: 3086 passed, 0 failed

Part of #126