proxy: remove dead state and broken fire-and-forget path (B1, D1)

B1: _inst_cumulative_tokens was written by pick_instance but never read
anywhere; delete the variable, global declaration, and per-call increment.
Load is already tracked via inst.ongoing_tokens.

D1: _send_prefill_async + the --fire-and-forget branch were unreachable
in practice (no launch/bench script enabled the flag) and broken even if
exercised: D-decode would fire before P registered the transfer_id,
guaranteeing a Mooncake 502. Collapse _handle_pd_sep to its synchronous
path and drop the CLI flag.

scripts/cache_aware_proxy.py

@@ -72,10 +72,6 @@ class InstanceState:
             self.cached_blocks = set(list(self.cached_blocks)[-100000:])
 
 
-# Cumulative token load per instance (for balanced session placement)
-_inst_cumulative_tokens: list[int] = []
-
-
 def _p_offload_penalty(inst: InstanceState) -> int:
     """Penalty for instances currently doing P-role offloaded prefills.
 
@@ -99,10 +95,6 @@ def pick_instance(instances: list[InstanceState], token_ids: list[int] | None,
     Instances doing P-role offloads get a large penalty to steer
     WARM/MEDIUM traffic away.
     """
-    global _inst_cumulative_tokens
-    if not _inst_cumulative_tokens:
-        _inst_cumulative_tokens = [0] * len(instances)
-
     avg_load = max(sum(i.ongoing_tokens for i in instances) / len(instances), 1.0)
 
     if session_id and session_id in affinity:
@@ -122,7 +114,6 @@ def pick_instance(instances: list[InstanceState], token_ids: list[int] | None,
             best_score = score
             best_idx = i
 
-    _inst_cumulative_tokens[best_idx] += input_length
     if session_id:
         affinity[session_id] = best_idx
     return instances[best_idx], best_idx
@@ -524,22 +515,6 @@ async def _handle_heavy_offload(api, req_data, headers, token_ids, input_length,
     return StreamingResponse(generate(), media_type="text/event-stream")
 
 
-async def _send_prefill_async(p_inst, api, prefill_data, p_headers, token_ids,
-                               input_length, breakdown):
-    """Fire-and-forget prefill: send and don't block caller."""
-    try:
-        resp = await p_inst.client.post(api, json=prefill_data, headers=p_headers)
-        breakdown["t_prefill_done"] = _time.monotonic()
-        resp.raise_for_status()
-        await resp.aclose()
-        p_inst.record_prefix(token_ids)
-    except Exception:
-        breakdown["t_prefill_done"] = _time.monotonic()
-        breakdown["prefill_error"] = True
-    finally:
-        p_inst.ongoing_tokens -= input_length
-
-
 async def _handle_pd_sep(api, req_data, request_id, token_ids, input_length,
                           session_id, headers):
     """PD-Sep mode with per-stage breakdown profiling."""
@@ -569,23 +544,19 @@ async def _handle_pd_sep(api, req_data, request_id, token_ids, input_length,
     p_inst.ongoing_tokens += input_length
     breakdown["t_prefill_sent"] = _time.monotonic()
 
-    if global_args.fire_and_forget:
-        asyncio.create_task(_send_prefill_async(
-            p_inst, api, prefill_data, p_headers, token_ids, input_length, breakdown))
-    else:
-        try:
-            resp = await p_inst.client.post(api, json=prefill_data, headers=p_headers)
-            breakdown["t_prefill_done"] = _time.monotonic()
-            resp.raise_for_status()
-            await resp.aclose()
-            p_inst.record_prefix(token_ids)
-        except Exception as e:
-            breakdown["t_prefill_done"] = _time.monotonic()
-            breakdown["prefill_error"] = True
-            _breakdown_log.append(breakdown)
-            raise HTTPException(status_code=502, detail=f"Prefill failed: {e}")
-        finally:
-            p_inst.ongoing_tokens -= input_length
+    try:
+        resp = await p_inst.client.post(api, json=prefill_data, headers=p_headers)
+        breakdown["t_prefill_done"] = _time.monotonic()
+        resp.raise_for_status()
+        await resp.aclose()
+        p_inst.record_prefix(token_ids)
+    except Exception as e:
+        breakdown["t_prefill_done"] = _time.monotonic()
+        breakdown["prefill_error"] = True
+        _breakdown_log.append(breakdown)
+        raise HTTPException(status_code=502, detail=f"Prefill failed: {e}")
+    finally:
+        p_inst.ongoing_tokens -= input_length
 
     # Send decode
     d_inst.ongoing_tokens += input_length
@@ -651,8 +622,6 @@ def parse_args():
                    help="PD-Sep prefill: URL [bootstrap_port]")
     p.add_argument("--decode", nargs=1, action="append", dest="decode_raw",
                    help="PD-Sep decode: URL")
-    p.add_argument("--fire-and-forget", action="store_true",
-                   help="Send prefill async, don't await before decode")
     p.add_argument("--heavy-threshold", type=int, default=20000,
                    help="New tokens threshold for HEAVY classification (adaptive offload)")
     p.add_argument("--offload", action="store_true",