CB-WP-0006 T08: control loop — 4 of 14 to 8 of 14, and a cost regression

Test 1: the enforced count rose. AM-2, AM-6, AM-9 and AM-11 moved from
unmutatable to red; AM-7's hash clause was re-earned so it is 2/3 rather
than 1/3. Kernel spec->code link 15/18 -> 18/18, names only.

Both denominators are stated. 8 of 14 is 57%, but four rows cannot be
enforced — AM-3 blocked on an artifact, AM-4c withdrawn, AM-5 declared
ungated by the spec, AM-10 withdrawn — so it is 8 of 10 enforceable. The
14 stays the headline and AM-4c stays in it on purpose: a score improved
by deleting the question is not an improvement.

Test 2: one row regressed and was caught. Moving AM-6's gate from debug to
release turned its mutation SURVIVED, because 4,000 black_box iterations
were calibrated against debug's 3.4x headroom and are invisible against
release's 20x. The generalizable finding is that a weak mutation is not a
fixed property of a row — it can become weak when the row's measurement
conditions change, without the row, the mutation or the code being
touched. Final SURVIVED count: 0.

Test 3 is now mechanical rather than asserted. mutation-check gained an
EXPECT-VACUOUS verdict: if a row's expect string appears in PASSING
output, the FA guard would accept any failure at all, so the row is
reported broken rather than red. Final run: 0 vacuous expects across 14
rows. The control exists because the failure happened — my first expect
for AM-2 was "AM-2", which appears in the passing report and would have
accepted a compile error as proof of enforcement.

The cost result is a refutation, not a win. Mechanical share rose to 50%,
the highest ever recorded and above the 38% baseline that motivated
CB-WP-0004. That is not a tooling regression: environment setup and task
closes are still at zero two passes on. It is the other half of CB-WP-0004
T06's finding arriving in force — text patching (45 turns, $13.79) and
orientation (19 turns, $10.97) never had their manual path removed, and a
code-heavy pass is exactly where that spends.

Mean context 493,486 against a 200,000 target, up from 315,170. SessionShape
has stated SS-01..SS-05 since CB-WP-0003 and none has ever been enforced —
the only acceptance-adjacent numbers in this project with no gate at all,
in a pass whose entire subject was ungated numbers.

Numbering corrected: the workplan said CB-EV-0004, which CB-WP-0005
already used. This is CB-EV-0005.

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

tools/mutation-check.py

@@ -282,10 +282,20 @@ def check_row(row):
 
     # Positive control 2: the baseline must be green, or "mutant red"
     # proves nothing.
-    base_ok, base_tail, _ = run(row.verify)
+    base_ok, base_tail, base_out = run(row.verify)
     if not base_ok:
         return "inconclusive", f"baseline already red: {base_tail}"
 
+    # CB-WP-0006 T08: the FA guard is only a guard if its `expect` string
+    # cannot appear in PASSING output. An expect of "AM-2" would match the
+    # normal report and accept any failure at all — which is how the guard
+    # goes vacuous without anyone noticing. My first attempt on AM-2 did
+    # exactly that.
+    if row.expect and row.expect in base_out:
+        return "EXPECT-VACUOUS", (
+            f"expect string {row.expect!r} appears in PASSING output, so it "
+            f"would accept any failure — the FA guard is inert for this row")
+
     try:
         mutated = original.replace(old, new, 1)
         # Positive control 3: the file content must actually differ.
@@ -337,12 +347,13 @@ def report(only=None):
                 detail = (f"{len(r.clauses) - len(unmet)}/{len(r.clauses)} "
                           f"clauses enforced")
         tally[verdict] = tally.get(verdict, 0) + 1
-        if verdict == "HARNESS-BROKEN":
+        if verdict in ("HARNESS-BROKEN", "EXPECT-VACUOUS"):
             broken.append(r.id)
         mark = {"red": "red        ", "SURVIVED": "SURVIVED   ",
                 "unmutatable": "unmutatable", "inconclusive": "inconclusive",
                 "PARTIAL": "PARTIAL    ",
                 "HARNESS-BROKEN": "BROKEN     ",
+                "EXPECT-VACUOUS": "EXPECT-VOID",
                 "WRONG-REASON": "WRONG-REASON"}[verdict]
         print(f"  [{mark}] {r.id:<6} {r.claim[:52]}")
         if detail:
@@ -366,8 +377,8 @@ def report(only=None):
         print(f"    ({withdrawn} withdrawn row(s) retained in the denominator "
               f"on purpose — a score\n     improved by deleting the question "
               f"is not an improvement)")
-    for k in ("PARTIAL", "SURVIVED", "WRONG-REASON", "unmutatable",
-              "inconclusive"):
+    for k in ("PARTIAL", "SURVIVED", "WRONG-REASON", "EXPECT-VACUOUS",
+              "unmutatable", "inconclusive"):
         if tally.get(k):
             print(f"    {k:<13} {tally[k]}")
     if only: