B3: load_only + sticky policies, capped-trace builder, sweep driver

Three additions land together because B3's whole point is comparing
LMetric against meaningful controls.

- scripts/cache_aware_proxy.py: two new --policy values.
  - load_only: pure min(num_requests) routing, no cache or affinity.
    The B3 control that strips locality so the LMetric-vs-load gap is
    legible.
  - sticky: first turn goes to min-load, subsequent turns ALWAYS
    return to the same instance, even under saturation. The B3
    control that maxes out locality so the hot-spot cost is legible.
- scripts/build_capped_trace.py: per-session turn cap (default 8).
  Generates the session-mass-equalized variant the TODO calls for so
  that hot-spot index can be re-measured with the heavy-tail removed.
- scripts/b3_sweep.sh: orchestrates the 5-cell sweep.
  - GPU_INDICES makes it easy to skip a dead GPU.
  - EXTRA_VLLM_ARGS defaults to --enable-prompt-tokens-details so
    usage.prompt_tokens_details.cached_tokens is populated. vLLM
    0.18.1 omits the field by default and breaks the reuse-decomp
    pipeline; the smoke run surfaced this.
  - Trap kills EngineCore by name in addition to "vllm serve" — the
    parent dies first but the child holds GPU memory. Was the root
    cause of the 89 GB ghost on GPU 0 earlier today.
  - Proxy readiness is a polling loop, not a fixed sleep.

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

scripts/build_capped_trace.py

@@ -0,0 +1,80 @@
+"""Cap per-session turn count to isolate session-mass effects in B3.
+
+Input trace is grouped by session_id (or reconstructed from
+parent_chat_id chains). Sessions with more than --max-turns turns are
+truncated to keep only their first N turns in trace order. The output
+preserves the original line ordering (timestamp order).
+"""
+
+from __future__ import annotations
+
+import argparse
+import json
+from collections import defaultdict
+from pathlib import Path
+
+
+def _resolve_session_id(row: dict, chat_to_session: dict[int, str]) -> str:
+    if "session_id" in row:
+        return str(row["session_id"])
+    cid = int(row["chat_id"])
+    pcid = int(row["parent_chat_id"])
+    if pcid < 0:
+        sid = str(cid)
+    else:
+        sid = chat_to_session.get(pcid, str(pcid))
+    chat_to_session[cid] = sid
+    return sid
+
+
+def main() -> None:
+    p = argparse.ArgumentParser(description="Cap per-session turn count")
+    p.add_argument("--input", type=Path, required=True)
+    p.add_argument("--output", type=Path, required=True)
+    p.add_argument("--max-turns", type=int, default=8,
+                   help="Keep at most N earliest turns per session")
+    args = p.parse_args()
+
+    chat_to_session: dict[int, str] = {}
+    kept: dict[str, int] = defaultdict(int)
+    rows: list[tuple[str, dict]] = []
+    with args.input.open("r", encoding="utf-8") as fh:
+        for line in fh:
+            line = line.strip()
+            if not line:
+                continue
+            row = json.loads(line)
+            sid = _resolve_session_id(row, chat_to_session)
+            row["session_id"] = sid
+            rows.append((sid, row))
+
+    in_n = len(rows)
+    sessions = len({sid for sid, _ in rows})
+
+    rows.sort(key=lambda x: (x[1]["session_id"], x[1].get("turn", 0)))
+    capped_rows: list[dict] = []
+    for sid, row in rows:
+        if kept[sid] >= args.max_turns:
+            continue
+        kept[sid] += 1
+        capped_rows.append(row)
+    capped_rows.sort(key=lambda r: r.get("timestamp", 0.0))
+
+    args.output.parent.mkdir(parents=True, exist_ok=True)
+    with args.output.open("w", encoding="utf-8") as fh:
+        for r in capped_rows:
+            fh.write(json.dumps(r) + "\n")
+
+    print(f"input rows: {in_n}, sessions: {sessions}")
+    print(f"capped rows: {len(capped_rows)} (max_turns={args.max_turns})")
+    dropped = in_n - len(capped_rows)
+    print(f"dropped:    {dropped} ({100 * dropped / max(in_n, 1):.1f}%)")
+    if capped_rows:
+        from collections import Counter
+        turns_dist = Counter(kept[s] for s in kept)
+        top = sorted(turns_dist.items())[:6]
+        print(f"turns/session (capped) sample: {top}")
+
+
+if __name__ == "__main__":
+    main()