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>

Makefile

@@ -83,7 +83,7 @@ am8:
 # one reading, because the noise multiplies rather than cancels.
 am7:
 	$(IN_REPO) $(CARGO) test --release -p games-ground --all-features \
-	  am7_scaling -- --ignored --nocapture --test-threads=1
+	  am7_cost_per_event -- --ignored --nocapture --test-threads=1
 
 # AM-9 peak RSS (fast, gated). AM-5 needs a clean build — see build-time.
 runtime-metrics: