B3 post-run helpers: engine_state slicer + per-policy aggregator

scripts/slice_engine_state.py filters a shared engine_*.jsonl by a
[t_start_unix, t_end_unix] window. Needed because the patched
scheduler appends to one file per engine across the whole sweep;
per-policy analysis requires the per-policy slice.

scripts/b3_analyze.sh drives the slice + joined_analysis loop for
every policy directory in a completed sweep, then aggregates one row
per policy (latency percentiles, APC, interference_index,
hotspot_index, reuse fractions, failure-cause counts) into
b3_policy_comparison.json.

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

scripts/slice_engine_state.py

@@ -0,0 +1,46 @@
+"""Slice shared engine_*.jsonl by a [t_start_unix, t_end_unix] window.
+
+Used between B3 policy runs and the analyzer so each policy gets
+its own per-window engine_state directory.
+"""
+
+from __future__ import annotations
+
+import argparse
+import json
+from pathlib import Path
+
+
+def main() -> None:
+    p = argparse.ArgumentParser()
+    p.add_argument("--input-dir", type=Path, required=True)
+    p.add_argument("--output-dir", type=Path, required=True)
+    p.add_argument("--window", type=Path, required=True,
+                   help="run_window.json with t_start_unix / t_end_unix")
+    args = p.parse_args()
+
+    window = json.loads(args.window.read_text())
+    ts = float(window["t_start_unix"])
+    te = float(window["t_end_unix"])
+
+    args.output_dir.mkdir(parents=True, exist_ok=True)
+    print(f"window {ts:.3f} .. {te:.3f}")
+    for src in sorted(args.input_dir.glob("engine_*.jsonl")):
+        n_in = n_out = 0
+        dst = args.output_dir / src.name
+        with src.open() as fi, dst.open("w") as fo:
+            for line in fi:
+                n_in += 1
+                try:
+                    r = json.loads(line)
+                except json.JSONDecodeError:
+                    continue
+                t = r.get("t_unix", 0)
+                if ts <= t <= te:
+                    fo.write(line)
+                    n_out += 1
+        print(f"  {src.name}: {n_out}/{n_in}")
+
+
+if __name__ == "__main__":
+    main()