Benchmark the startup claim, and write down what a run is

The milestone is measured on being lighter than Kedro, so make bench-startup
measures it rather than asserting it: 61 ms from submit to result against
1110 ms for kedro run on a pipeline that does the same nothing. The difference
is not orchestration, it is that nothing is booted per run — on a 510-config
sweep that is about nine minutes of pure startup that never happens.

docs/flows/runs.md is the guide: batch flows, sweeps, reporting from inside a
node, artifacts, and the two sanctioned patterns for objects that cannot be
serialized — keep them in one node, or cross at a checkpoint.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AD8SfVhzXBG2nAfFcVh3iD

Makefile

@@ -3,7 +3,7 @@
 # The workspace root delegates to these (see ../Makefile).
 
 .PHONY: dev-utils dev dev-local up down update install dev-backend dev-frontend \
-        generate-client seed-example test test-backend test-frontend soak lint lint-backend \
+        generate-client seed-example test test-backend test-frontend soak bench-startup lint lint-backend \
         lint-frontend umami clean help
 
 COMPOSE_ROOT := $(CURDIR)
@@ -114,6 +114,9 @@ test-frontend:  ## Run frontend tests (Playwright e2e)
 soak:  ## Run the soak/chaos harness (SOAK_ARGS="--minutes 30 --scenario redis")
 	cd backend && uv run python scripts/soak.py $(SOAK_ARGS)
 
+bench-startup:  ## Time submitting a run (BENCH_ARGS="--kedro ../some/kedro/project")
+	cd backend && uv run python scripts/bench_startup.py $(BENCH_ARGS)
+
 # ── Linting ───────────────────────────────────────────────────────
 
 lint: lint-backend lint-frontend  ## Run all linters