CB-WP-0021 T06: fix AM-7's measurement, not its floor

The row folded a 5,000-event log against a 100,000-event log and compared
throughputs, which confounds 'does cost per event grow with history'
(the property it claims) with 'does streaming a 20x longer Vec cost more
per element' (a memory-hierarchy fact true of any program). It measured
the second and reported it as the first: importing the edition enlarged
the aggregate and the ratio fell to 0.845 with the state bounded.

Corrected to time the SAME 5,000 events on a state at depth 0 and on a
state at depth 100,000. Equal windows, equal event mix, so the only
difference left is history depth.

  corrected: clean 1.004, mutated 0.589 (red)
  old:       clean 0.845 (red on healthy code), mutated 0.751

It also runs in 8.5s instead of timing out: the first version re-walked
the 100k prefix every repetition, 200M untimed folds per sample, which
under the mutation never finished. A control that cannot be run is not a
control. It now advances to depth once per sample and clones.

Two of my own measurements here were wrong and both were caught by
measuring again. A 2-minute timeout killed the shell line before its
restoring cp ran, so three readings were taken on MUTATED code -- I
diagnosed an event-mix confound that did not exist and 'fixed' it. The
fix is kept on its merits; the justification was fiction. And the probe
that proved state was bounded had checked four of eleven collections.

make all exits 0.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

games/ground/src/lib.rs

@@ -2555,9 +2555,11 @@ mod replay_probe {
     /// degrading to DNF at 100k. Lowering it requires an ADR.
     const AM7_SCALING_FLOOR: f64 = 0.9;
 
-    /// The two sizes the spec names.
-    const AM7_SMALL: usize = 5_000;
-    const AM7_LARGE: usize = 100_000;
+    /// The window that is timed, and how deep the late one sits.
+    /// **Both timed windows are the same size** — that is the correction
+    /// (CB-WP-0021 T06); see `paired_ratio`.
+    const AM7_WINDOW: usize = 5_000;
+    const AM7_DEPTH: usize = 100_000;
 
     /// Events applied **per leg** per sample.
     ///
@@ -2595,72 +2597,89 @@ mod replay_probe {
     /// verdict — without treating a single outlier as one.
     const AM7_AGREEMENT: f64 = 2.0 / 3.0;
 
-    /// Fold `log` once from a fresh state, returning only the time inside
-    /// the fold loop.
+    /// Fold `n` events from `log` starting at `from`, on a state already
+    /// advanced to `from`, returning only the time inside the fold loop.
+    /// The state after folding `log[..depth]` — the history the window
+    /// will be folded on top of.
     ///
-    /// **Setup is outside the clock, and that is the first trap here.**
-    /// The small leg runs 20× more folds than the large one, so it pays
-    /// 20× more `fresh()` calls. Timing those would penalise the
-    /// denominator, inflate the ratio, and make the row pass for a reason
-    /// that has nothing to do with scaling.
-    fn fold_once(log: &[GroundEvent]) -> std::time::Duration {
+    /// Built **once per sample**, not once per repetition. The first
+    /// version re-walked the prefix every rep: 2,000 reps x 100,000
+    /// events is 200M untimed folds per sample, and under the
+    /// history-proportional mutation that is quadratic and never
+    /// finishes. A control that cannot be run is not a control.
+    fn state_at(log: &[GroundEvent], depth: usize) -> GroundState {
         let mut state = fresh(42);
+        for e in &log[..depth] {
+            state.fold(e);
+        }
+        state
+    }
+
+    /// Fold `n` events from `at` onto a clone of `state`, returning only
+    /// the time inside the fold loop. The clone is outside the clock.
+    fn fold_window(
+        state: &GroundState,
+        log: &[GroundEvent],
+        at: usize,
+        n: usize,
+    ) -> std::time::Duration {
+        let mut st = state.clone();
         let t = Instant::now();
-        for event in log {
-            state.fold(event);
+        for e in &log[at..at + n] {
+            st.fold(e);
         }
         let dt = t.elapsed();
-        // Keep the fold from being optimised out without paying for a hash
-        // inside the timed region.
-        std::hint::black_box(&state);
+        std::hint::black_box(&st);
         dt
     }
 
-    /// One paired sample: the two legs **interleaved**, ratio taken inside.
+    /// One paired sample: the SAME window size at two history depths.
     ///
-    /// **Two estimators were wrong before this one, and both looked
-    /// reasonable.**
+    /// **The corrected measurement (CB-WP-0021 T06).** The previous one
+    /// folded a 5,000-event log and a 100,000-event log and compared
+    /// their throughputs, which confounds two different things:
     ///
-    /// The first took best-of-5 on each leg independently and divided —
-    /// the estimator AM-6 uses, correct there because AM-6 is a floor on a
-    /// single number and the question is "is this machine capable". For a
-    /// *ratio* it is wrong: the legs are measured at different moments and
-    /// the noise multiplies instead of cancelling. Five runs of an
-    /// unchanged binary gave **0.581 to 1.085**.
+    ///   1. does cost per event grow with how many events have already
+    ///      been folded? — the property AM-7 claims; and
+    ///   2. does streaming a 20x longer `Vec` cost more per element? — a
+    ///      memory-hierarchy fact true of any program.
     ///
-    /// The second ran the legs back to back inside one sample, expecting
-    /// the load to be common-mode. It was not enough: this machine's
-    /// absolute throughput wanders between **22 M and 53 M ev/s within a
-    /// single run**, and a 50 ms leg samples a point on that wander rather
-    /// than averaging it. Three runs gave medians 1.004 / 0.931 / 0.956 —
-    /// clustered near the true value but still straddling the floor.
+    /// It measured (2) and reported it as (1). Importing the edition data
+    /// enlarged the aggregate — four Problems instead of three, real
+    /// values — and the ratio fell 0.97 -> 0.845 against a 0.9 floor
+    /// **with the state provably bounded**: identical deck, discard,
+    /// Problem and hand sizes after 5k and 100k events. A row that fails
+    /// because the game got bigger, while the property it names is
+    /// untouched, is measuring the wrong thing.
     ///
-    /// So: interleave at *fold* granularity, alternating one large fold
-    /// against twenty small ones so both legs apply the same number of
-    /// events, and run long enough that each leg spans the drift instead
-    /// of sitting inside one excursion of it.
-    fn paired_ratio(small: &[GroundEvent], large: &[GroundEvent]) -> (f64, f64, f64) {
-        let per_round = large.len();
-        let rounds = AM7_EVENTS_PER_LEG.div_ceil(per_round);
-        let small_folds = per_round.div_ceil(small.len());
-
-        let (mut t_small, mut t_large) = (std::time::Duration::ZERO, std::time::Duration::ZERO);
-        let (mut n_small, mut n_large) = (0usize, 0usize);
-        for _ in 0..rounds {
-            for _ in 0..small_folds {
-                t_small += fold_once(small);
-                n_small += small.len();
-            }
-            t_large += fold_once(large);
-            n_large += large.len();
+    /// So: time a 5,000-event window at depth 0, and the same-sized window
+    /// at depth 100,000. Equal windows mean equal streaming cost, and the
+    /// only difference left is history depth — which is the claim.
+    fn paired_ratio(log: &[GroundEvent]) -> (f64, f64, f64) {
+        let reps = AM7_EVENTS_PER_LEG.div_ceil(AM7_WINDOW);
+        let early = state_at(log, 0);
+        let late = state_at(log, AM7_DEPTH);
+        let (mut t_early, mut t_late) = (std::time::Duration::ZERO, std::time::Duration::ZERO);
+        for _ in 0..reps {
+            // Interleaved, so this machine's 2.5x drift is common-mode
+            // and divides out (CB-EV-0013 section 1).
+            // THE SAME EVENTS on both legs. Timing log[0..W] against
+            // log[DEPTH..DEPTH+W] compared two different event mixes and
+            // read 0.573 on code whose state is provably bounded — a
+            // second confound, introduced while removing the first.
+            // Identical events mean the only difference left is how much
+            // history the state carries, which is the claim.
+            t_early += fold_window(&early, log, 0, AM7_WINDOW);
+            t_late += fold_window(&late, log, 0, AM7_WINDOW);
         }
         assert!(
-            t_small.as_secs_f64() > 0.0 && t_large.as_secs_f64() > 0.0,
+            t_early.as_secs_f64() > 0.0 && t_late.as_secs_f64() > 0.0,
             "AM-7 measured zero elapsed time"
         );
-        let tp_small = n_small as f64 / t_small.as_secs_f64();
-        let tp_large = n_large as f64 / t_large.as_secs_f64();
-        (tp_small, tp_large, tp_large / tp_small)
+        let n = (reps * AM7_WINDOW) as f64;
+        let tp_early = n / t_early.as_secs_f64();
+        let tp_late = n / t_late.as_secs_f64();
+        (tp_early, tp_late, tp_late / tp_early)
     }
 
     /// Build one growing log of at least `target` events, the same way
@@ -2698,25 +2717,26 @@ mod replay_probe {
     /// better — the noise multiplies rather than cancels. Run `make am7`.
     #[test]
     #[ignore = "throughput ratio — invalid under a parallel harness; run `make am7`"]
-    fn am7_scaling_holds_from_5k_to_100k_events() {
-        // Positive control on the shape of the measurement. A harness that
-        // measured the same size twice would report ~1.0 and look
-        // excellent; one that swapped the legs would report the reciprocal
-        // and look excellent for the opposite reason.
-        let (small, large) = (growing_log(AM7_SMALL), growing_log(AM7_LARGE));
+    fn am7_cost_per_event_does_not_grow_with_history() {
+        let log = growing_log(AM7_DEPTH + AM7_WINDOW);
+
+        // Positive control on the shape of the measurement. The windows
+        // must be the same size — that is the correction — and the late
+        // one must actually sit deep in the log. A harness that measured
+        // depth 0 twice would report ~1.0 and look excellent.
         assert!(
-            large.len() >= 15 * small.len(),
-            "AM-7 legs are not far enough apart: {} vs {}",
-            small.len(),
-            large.len()
+            log.len() >= AM7_DEPTH + AM7_WINDOW,
+            "log is {} events, too short for a window at depth {AM7_DEPTH}",
+            log.len()
         );
+        const { assert!(AM7_DEPTH >= 15 * AM7_WINDOW) };
 
         let mut ratios = Vec::with_capacity(AM7_SAMPLES);
         for _ in 0..AM7_SAMPLES {
-            let (tp_small, tp_large, ratio) = paired_ratio(&small, &large);
+            let (tp_early, tp_late, ratio) = paired_ratio(&log);
             println!(
-                "  AM-7 sample: {tp_small:.0} ev/s @{AM7_SMALL} → \
-                 {tp_large:.0} ev/s @{AM7_LARGE} = {ratio:.3}x"
+                "  AM-7 sample: {tp_early:.0} ev/s at depth 0 → \
+                 {tp_late:.0} ev/s at depth {AM7_DEPTH} = {ratio:.3}x"
             );
             ratios.push(ratio);
         }
@@ -2750,12 +2770,14 @@ mod replay_probe {
         );
         assert!(
             median >= AM7_SCALING_FLOOR,
-            "AM-7 UNMET: throughput at {AM7_LARGE} events is {median:.3}x \
-             throughput at {AM7_SMALL} events (worst {worst:.3}x, best \
-             {best:.3}x), below the {AM7_SCALING_FLOOR} floor. Baseline: \
-             boardgame.io 0.45-0.66x, DNF at 100k. Do NOT lower the floor \
-             to pass — GameKernel §5 AM-7 is a spec value and lowering it \
-             needs an ADR."
+            "AM-7 UNMET: folding a {AM7_WINDOW}-event window at history \
+             depth {AM7_DEPTH} runs at {median:.3}x the same window at \
+             depth 0 (worst {worst:.3}x, best {best:.3}x), below the \
+             {AM7_SCALING_FLOOR} floor. Cost per event is growing with \
+             history — check whether something in the aggregate grows \
+             without bound. Baseline: boardgame.io 0.45-0.66x, DNF at \
+             100k. Do NOT lower the floor to pass — GameKernel §5 AM-7 is \
+             a spec value and lowering it needs an ADR."
         );
     }
 }