The hot-sweep variant of B3 writes one shared engine_state across
all policies; the isolated variant writes per-policy. Previously
slice_engine_state.py was called unconditionally and would
overwrite an isolated policy's real data with an empty slice (the
isolated policy's run-window doesn't overlap with the shared dir's
contents).
Now we check the policy directory's engine_state for any non-empty
engine_*.jsonl first; if present, use it directly; else slice from
the shared one as before.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>