Keep the ablation arms inside the line budget the repo enforces

copy_store() moved into bench/scratch.py: systems.py had reached 512 lines
and bench/filesize_check fails anything over 500. The move is behaviour
identical, so the scale reports keep the commit stamp they were generated
against — the code they describe is complete at that commit.

bench/scratch.py

@@ -0,0 +1,24 @@
+"""Every harness measures a copy. The frozen baseline store is cited evidence, and the product
+migrates whatever database it opens."""
+
+from __future__ import annotations
+
+import shutil
+import time
+from pathlib import Path
+
+
+def copy_store(home: Path, scratch: Path) -> tuple[Path, float]:
+    """Work on a copy: the baseline store is frozen evidence and must not be migrated in place."""
+    source = home / "data" / "ely_eye.sqlite"
+    if not source.is_file():
+        raise SystemExit(
+            f"{source}: no product database, build it with python -m bench.ingest_baseline"
+        )
+    data = scratch / "data"
+    data.mkdir(parents=True, exist_ok=True)
+    began = time.perf_counter()
+    for suffix in ("", "-wal", "-shm"):
+        if (path := Path(str(source) + suffix)).is_file():
+            shutil.copyfile(path, data / (source.name + suffix))
+    return scratch, time.perf_counter() - began