Let every harness work on a copy of the frozen store

copy_store lived in dense_scale, so the two harnesses that also needed it reached back into a
measurement module for a plumbing helper. It now sits in systems beside the product path it
protects, and bench.run passes a scratch directory to the live check: the product path migrates
whatever database it opens, and the frozen baseline is cited evidence.

bench/term_sweep.py

@@ -16,12 +16,11 @@ import statistics
 from pathlib import Path
 from typing import Any
 
-from bench.dense_scale import copy_store
 from bench.metrics import aggregate, score_query
 from bench.query_set import load_queries, scale_probes
 from bench.report import git_commit
 from bench.run import latency
-from bench.systems import live_product_rankings, machine_facts
+from bench.systems import copy_store, live_product_rankings, machine_facts
 
 FRACTIONS = [0.20, 0.10, 0.05, 0.02, 0.01, 0.005]
 TOP_K = 10