test(causal): controller test no longer races the sweep's site snapshot

run_experiments snapshots the site registry once at entry. stage-x is
registered lazily (first causal_site! execution in the worker), so on a
1-core box the snapshot deterministically wins whenever a sibling test
has already paid the tsc_hz calibration — stage-x missed the sweep and
the summary assert tripped (silently, pre-propagation; the earlier
cascade attribution was incomplete). The worker now signals after its
first site entry and the test waits on it before starting the sweep.
Jarred separately: the lazy-registration trap is a lib UX hazard worth a
doc note or warm-up guidance.

tests/causal.rs

@@ -310,7 +310,14 @@ fn controller_produces_report() {
     smarm::init(smarm::Config::exact(2)).run(|| {
         let stop = Arc::new(AtomicBool::new(false));
         let stop2 = stop.clone();
+        // run_experiments snapshots the site registry once at entry; the
+        // worker must have executed its first `causal_site!` (lazy
+        // registration) before the call, or stage-x misses the sweep — a
+        // race this test loses deterministically on a 1-core box whenever a
+        // sibling test has already paid the tsc_hz calibration.
+        let (ready_tx, ready_rx) = smarm::channel::<()>();
         let worker = smarm::spawn(move || {
+            let mut ready = Some(ready_tx);
             while !stop2.load(Ordering::Relaxed) {
                 {
                     let _g = smarm::causal_site!("stage-x");
@@ -319,8 +326,12 @@ fn controller_produces_report() {
                     }
                 }
                 smarm::progress!("items");
+                if let Some(tx) = ready.take() {
+                    let _ = tx.send(());
+                }
             }
         });
+        ready_rx.recv().unwrap();
 
         let results = smarm::causal::run_experiments(&smarm::causal::ExperimentPlan {
             speedups_pct: vec![0, 50],