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>

tools/mutation-check.py

@@ -231,7 +231,7 @@ def rows():
                  "and gates the median at 0.9. Measured 0.956-1.068 over "
                  "three runs; the mutation drives it to 0.751.",
                  (CARGO + ["test", "--release", "-p", "games-ground",
-                           "--all-features", "am7_scaling", "--",
+                           "--all-features", "am7_cost_per_event", "--",
                            "--ignored", "--test-threads=1"],
                   ("games/ground/src/lib.rs",
                    "    fn fold(&mut self, event: &Self::Event) {\n"