Give the engine bench a make target

And stop its drain loop asking for the whole of `stats()` every twenty
milliseconds — four round trips a poll, competing with the engine for the
connection it is measuring.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BpfSinyCBfjuieikyfMPbf

Makefile

@@ -148,6 +148,12 @@ soak:  ## Run the soak/chaos harness (SOAK_ARGS="--minutes 30 --scenario redis")
 bench-startup:  ## Time submitting a run (BENCH_ARGS="--kedro ../some/kedro/project")
 	cd backend && uv run python scripts/bench_startup.py $(BENCH_ARGS)
 
+# In-process, so it needs no stack: what a message costs the engine in work
+# and in time. `BENCH_ARGS="--redis <host>"` measures it against a real Redis,
+# which is where the round trips are.
+bench-engine:  ## Engine throughput and per-message cost (BENCH_ARGS="--redis localhost")
+	cd backend && uv run python scripts/bench_engine.py $(BENCH_ARGS)
+
 # ── Linting ───────────────────────────────────────────────────────
 
 build:  ## Build the fluksio and fluksio-worker wheels into dist/