CB-WP-0006 T02: instrument AM-2; report AM-3 blocked, with the argument

Both rows were unmutatable for the same stated reason. They resolved
differently, and the difference is the point.

AM-2 is instrumented and enforced — tools/size-metrics.py, in `make all`:

  AM-2: 27.2 LOC/rule  [ok  target <= 40]  (1.47x headroom)
        1,575 impl lines / 58 rules

Tests are excluded because AM-2 asks what a rule costs, not how much it is
exercised; lib.rs is ~18% test code and including it would have flattered
the number. This matters because AM-2 is AM-1's anti-gaming pair: 100%
rule coverage means nothing if the rules are trivially small, and AM-1 has
been reported met since CB-WP-0001 with its pair uninstrumented.

Verified red by a property mutation — ~800 lines of filler injected into
the impl, pushing the ratio past 40 — not a threshold tweak. The expect
string is the precise failure signature "FAIL target <= 40"; my first
attempt used "AM-2", which also matches passing output and would have
made the FA guard vacuous.

AM-3 is BLOCKED, not uninstrumented, and that is a finding rather than a
deferral. It measures LOC to express the CB-RES-0001 synthetic game on our
kernel, against a boardgame.io baseline of ~36 LOC for a declarative 3p
commit/reveal game object. That artifact has never been built: games/
contains only ground, and benches/synthetic.rs drives GROUND rather than
defining a synthetic game. Measuring GROUND's 1,575 impl lines against a
36-line synthetic game object would compare two different games and call
the difference a D1 result.

So the tool ships the measurement — a marker-delimited region, self-tested
— and reports the row blocked, naming the missing artifact. A number would
have been worse than a blank. It stays unmutatable and still counts
against M-D1-MUT per ADR-0005 §1: a row that cannot fail asserts nothing,
however good the reason.

M-D1-MUT: 5 -> 6 of 14.

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

tools/mutation-check.py

@@ -88,15 +88,32 @@ def rows():
             mutate=("scenarios/ground/gr-r06-round-resolve.yaml",
                     "GR-R06, ", "")),
 
+        # A PROPERTY mutation: inflate the implementation past the ratio.
+        # 58 rules x 40 = 2,320; the impl is 1,575, so ~800 lines of filler
+        # crosses it. Raising the threshold instead would only prove the
+        # comparison runs.
         Row("AM-2", "<= 40 spec lines per rule in games/ground",
-            unmutatable="no instrument computes it. `make loc` prints LOC "
-                        "and nothing divides by rule count or compares to 40; "
-                        "CB-EV-0001 records AM-2 as unreported."),
+            verify=py + ["tools/size-metrics.py"],
+            mutate=("games/ground/src/lib.rs",
+                    "impl Aggregate for GroundState {",
+                    "fn _am2_filler() {\n" + "    let _x = 0;\n" * 800
+                    + "}\n\nimpl Aggregate for GroundState {"),
+            expect="FAIL target <= 40"),
 
         Row("AM-3", "synthetic workload definition <= 50 LOC",
-            unmutatable="no instrument. The synthetic workload is hardcoded "
-                        "in benches/synthetic.rs (see K18) and its LOC is "
-                        "never measured or compared."),
+            unmutatable="BLOCKED, not uninstrumented — the distinction "
+                        "matters. `make size-metrics` ships the measurement "
+                        "(a marker-delimited region) and reports the row "
+                        "blocked, because the artifact it measures has never "
+                        "been built: games/ contains only ground, and "
+                        "benches/synthetic.rs drives GROUND rather than "
+                        "defining a synthetic game. The baseline is a "
+                        "declarative 3p commit/reveal game object (~36 LOC, "
+                        "boardgame.io); measuring GROUND's 1,575 impl lines "
+                        "against it would compare two different games and "
+                        "call the difference a D1 result. Still counts "
+                        "against M-D1-MUT (ADR-0005 §1) — a row that cannot "
+                        "fail asserts nothing, however good the reason."),
 
         Row("AM-4a", "third-party LOC, shipped runtime <= 250,000",
             verify=py + ["tools/dep-weight.py"],