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>

workplans/CB-WP-0021-import-the-edition.md

@@ -2,7 +2,7 @@
 id: CB-WP-0021
 kind: product
 title: "Import the edition: the game plays its own data"
-status: ready
+status: active
 state_hub_workstream_id: "782b1c37-f3a7-469b-87a3-fa73ebe758d2"
 ---
 
@@ -114,7 +114,7 @@ alternatives to take, and that is the ADR.
 
 ```task
 id: CB-WP-0021-T01
-status: todo
+status: done
 priority: high
 state_hub_task_id: "68e4fe63-eec6-4fb8-a84f-32c7edee19af"
 ```
@@ -155,7 +155,7 @@ replay determinism, is not a data-loading change.
 
 ```task
 id: CB-WP-0021-T05
-status: todo
+status: done
 priority: high
 state_hub_task_id: "c77c0b39-0841-40bf-8078-135486d7ed55"
 ```
@@ -182,7 +182,7 @@ numbers nobody ruled on.
 
 ```task
 id: CB-WP-0021-T02
-status: todo
+status: done
 priority: high
 state_hub_task_id: "28c3ff2c-16ae-47b5-9474-10e754936c60"
 ```
@@ -207,7 +207,7 @@ Hidden → face down) and `hidden_priority`.
 
 ```task
 id: CB-WP-0021-T03
-status: todo
+status: done
 priority: medium
 state_hub_task_id: "ff3bd923-9066-49ce-aadd-a3552e4964ff"
 ```
@@ -228,6 +228,51 @@ ruled. Message `ground-game` with the outcome.
 Do **not** quietly delete a failing-in-fact scenario. CB-EV-0005: *a score
 improved by deleting the question is not an improvement.*
 
+## Task: fix AM-7's measurement, not its floor
+
+```task
+id: CB-WP-0021-T06
+status: done
+priority: high
+```
+
+T05 turned AM-7 red: median **0.845** against a 0.9 floor, all nine
+samples below. The maintainer chose to **fix the measurement** rather than
+ADR the floor or optimise the fold.
+
+**The row was measuring the wrong thing.** It folded a 5,000-event log and
+a 100,000-event log and compared throughputs, which confounds:
+
+1. does cost per event grow with how many events have been folded? — the
+   property AM-7 claims; and
+2. does streaming a 20× longer `Vec` cost more per element? — a
+   memory-hierarchy fact true of any program.
+
+It measured (2) and reported it as (1). Importing the edition enlarged the
+aggregate and the ratio fell, **with the state bounded**.
+
+**Corrected:** time a 5,000-event window on a state at depth 0, and *the
+same events* on a state at depth 100,000. Equal windows, equal event mix;
+the only difference left is history depth.
+
+| | clean | history-proportional mutation |
+|---|---|---|
+| corrected | **1.004** | **0.589 — red** |
+| old | 0.845 (red on healthy code) | 0.751 |
+
+Renamed `am7_cost_per_event_does_not_grow_with_history`, because the old
+name described the confounded measurement.
+
+**Two of my own measurements in this task were wrong, and both were caught
+by measuring again.** A 2-minute timeout killed the shell line before its
+restoring `cp` ran, so the next three readings were taken on **mutated
+code** — I diagnosed a "second confound" from event-mix that did not
+exist, and "fixed" it by folding identical events on both legs. That
+change is kept, on its own merits: identical events remove a real
+potential confound. But the justification I gave for it was fiction, and
+the probe that proved state was bounded had only checked four of eleven
+collections.
+
 ## Task: evidence
 
 ```task