Add unified_nixl_both policy: NIXL connector isolation control

Adds a NIXL-backed counterpart to unified_kv_both so we can attribute
the kv_both substrate overhead measured in the elastic_migration_v2
section to either Mooncake-specific code or a generic v1-connector
cost shared by all connectors.

- scripts/cache_aware_proxy.py: register --policy unified_nixl_both.
  Picker is identical to unified (and unified_kv_both); routing
  decisions never go through the PD-sep branch. Differs only at the
  vLLM launch layer.
- scripts/b3_isolated_policy.sh: new KV_CONNECTOR env var
  (Mooncake|Nixl), auto-set based on POLICY. NIXL launch path uses
  --kv-transfer-config '{"kv_connector":"NixlConnector","kv_role":"kv_both"}'
  with no VLLM_MOONCAKE_BOOTSTRAP_PORT (NIXL uses UCX side-channels).
- Health-check timeout: 90 iterations * 2s -> 180 iterations * 2s
  (180s -> 360s). Empirically NIXL needs ~100-150s per instance to
  initialize the UCX agent and register KV cache memory; 8
  concurrent NIXL launches frequently overshoot the previous 180s
  budget. Mooncake is unaffected (still finishes well inside the new
  budget). The 8-vLLM unified_nixl_both first launch tripped the
  old timeout despite 7/8 instances reaching startup-complete.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

scripts/cache_aware_proxy.py

@@ -762,6 +762,9 @@ async def lifespan(app: FastAPI):
 
         # Bootstrap combined instances for offload (need engine_ids for KV transfer)
         policy = getattr(global_args, 'policy', 'linear')
+        # Mooncake-based modes still need bootstrap discovery; NIXL uses
+        # its own UCX side-channel and doesn't go through our proxy
+        # bootstrap path (and unified_nixl_both never PD-seps anyway).
         needs_bootstrap = (
             global_args.offload
             or policy in ("unified_v2", "unified_kv_both")
@@ -927,11 +930,16 @@ async def _handle_combined(api, req_data, token_ids, input_length, session_id, h
         chosen, best_idx = pick_instance_sticky(
             combined_instances, token_ids, session_id, input_length,
             session_affinity_combined)
-    elif policy == "unified" or policy == "unified_kv_both":
+    elif policy in ("unified", "unified_kv_both", "unified_nixl_both"):
         # unified_kv_both: same picker as `unified`, but the vLLMs are
-        # launched in kv_role=kv_both. Use this as an isolation control
-        # for `unified_v2` so the v2-vs-v1 gap reflects only the PD-sep
-        # branch, not the kv_both always-on overhead.
+        # launched in kv_role=kv_both with MooncakeConnector. Use this
+        # as an isolation control for `unified_v2` so the v2-vs-v1 gap
+        # reflects only the PD-sep branch, not the kv_both always-on
+        # overhead.
+        # unified_nixl_both: identical to unified_kv_both but with
+        # NixlConnector at the vLLM layer. Used to attribute the
+        # kv_both overhead to either Mooncake-specific code or a
+        # generic v1-connector cost.
         chosen, best_idx, decision = pick_instance_unified_hybrid(
             combined_instances, token_ids, session_id, input_length,
             session_affinity_combined)
@@ -1291,13 +1299,17 @@ def parse_args():
                    help="Comma-separated bootstrap ports for combined instances (for offload mode)")
     p.add_argument("--policy", type=str, default="linear",
                    choices=["linear", "lmetric", "load_only", "sticky",
-                             "unified", "unified_kv_both", "unified_v2"],
+                             "unified", "unified_kv_both",
+                             "unified_nixl_both", "unified_v2"],
                    help="Routing policy: linear (cache-aware), lmetric (P_tokens × BS), "
                         "load_only (B3 control: pure min-num_requests), "
                         "sticky (B3 control: hard session affinity), "
                         "unified (hybrid affinity + LMetric fallback), "
-                        "unified_kv_both (unified on kv_both vLLMs; isolation "
-                        "control for unified_v2; PD-sep never triggers), "
+                        "unified_kv_both (unified picker on kv_both Mooncake "
+                        "vLLMs; isolation control for unified_v2), "
+                        "unified_nixl_both (same as unified_kv_both but using "
+                        "NixlConnector instead of MooncakeConnector; isolates "
+                        "connector implementation from policy effect), "
                         "or unified_v2 (unified + selective per-request PD-sep "
                         "via Mooncake; requires --bootstrap-ports and "
                         "kv_role=kv_both vLLM launch)")