Derive a unique per-job seed for minicalosim shard generation

Concurrent job launches in create_root_files.py can start within the
same wall-clock second, and minicalosim's default seed falls back to
time(NULL) in that case — so two "independent" shards could silently
get identical RNG state and produce byte-identical physics. Requires
the companion MINICALOSIM_SEED env-var support in the minicalosim repo.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

scripts/create_root_files.py

@@ -25,6 +25,7 @@ import shutil
 import subprocess
 import sys
 import uuid
+import zlib
 from dataclasses import dataclass
 from concurrent.futures import ThreadPoolExecutor, as_completed
 from pathlib import Path
@@ -104,6 +105,20 @@ def plan_jobs(
     return jobs
 
 
+def job_seed(kind: str, gen: str, job: SimJob) -> int:
+    """Deterministic RNG seed for one sim job, unique per (kind, gen, detector, config, shard).
+
+    Jobs run concurrently (ThreadPoolExecutor below) and can start within the
+    same wall-clock second; minicalosim's default seed falls back to
+    time(NULL) in that case, so two concurrently-launched jobs can silently
+    get identical RNG state and produce byte-identical physics despite
+    landing in separate shard files. Deriving the seed from the full job
+    identity instead keeps it both unique and reproducible.
+    """
+    key = f"{kind}|{gen}|{job.detector}|{job.config or ''}|{job.shard_index}"
+    return zlib.crc32(key.encode()) & 0x7FFFFFFF
+
+
 def run_job(
     job: SimJob,
     executable: Path,
@@ -124,7 +139,8 @@ def run_job(
         cmd.append(job.config)
     cmd.append(str(events_per_file))
 
-    result = subprocess.run(cmd, cwd=workdir, capture_output=True, text=True)
+    env = dict(os.environ, MINICALOSIM_SEED=str(job_seed(kind, gen, job)))
+    result = subprocess.run(cmd, cwd=workdir, capture_output=True, text=True, env=env)
 
     if result.returncode != 0:
         return JobResult(
@@ -254,7 +270,8 @@ def run_make_root(
             / job.detector
             / f"shard-{job.shard_index:03d}.root"
         )
-        print(f"  {' '.join(cmd)}  ->  {dest}")
+        seed = job_seed(kind, gen, job)
+        print(f"  MINICALOSIM_SEED={seed} {' '.join(cmd)}  ->  {dest}")
 
     if not execute:
         print("\nDry run only — pass --execute to apply.")