test(accounting): the judge must SAY it proves carriage and not fidelity (red)

1 of 1 red, on an ASSERT about the published sentence. The behavioural half of
the same test is already green and that is the point: PM's M10 measured that a
mutated converter writing a BLANK PNG gives `asset_holds = True`, because the
bundle is internally consistent -- the digest it claims to have written really
is the asset's digest -- and the judge has no opinion about whether those bytes
hold the source's picture.

The limit is therefore asserted in both directions: it is REAL (the blank PNG
is accepted here) and it must be STATED (the docstring names what the route
cannot see). "A bundle claiming a conversion it did not perform still fails"
reads wider than the route reaches; the suite fells that mutant, this gate
cannot.

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

tests/test_accounting_gate.py

@@ -671,6 +671,42 @@ def test_the_build_never_writes_a_claim_the_document_supplied(tmp_path: Path) ->
     )
 
 
+def test_the_judge_proves_carriage_and_says_it_does_not_prove_fidelity(tmp_path: Path) -> None:
+    """The limit, MEASURED here rather than trusted to the prose beside it.
+
+    PM's M10 2026-09-19: a mutated converter that writes a BLANK PNG gives
+    `asset_holds = True`. The bundle is internally consistent -- the digest it
+    claims to have written really is the asset's digest -- and the judge has
+    no opinion about whether those bytes hold the source's picture. The suite
+    fells that mutant (`test_the_carried_png_holds_the_source_pixels_exactly`
+    decodes both sides); this gate cannot, and the docstring's "a bundle
+    claiming a conversion it did not perform still fails" reads wider than
+    the route reaches.
+
+    So the limit is asserted in BOTH directions: it is real (the blank PNG is
+    accepted) and it is stated (the docstring names it). Teaching the judge
+    pixels is a different job; leaving a reader to infer the gap is not.
+    """
+    source = tmp_path / "figur.bmp"
+    source.write_bytes(_huge_bmp())
+    blank = tmp_path / "blank.png"
+    blank.write_bytes(_png_bytes(b"\xff\xff\xff"))
+    before = gate._sha256(source)
+    after = gate._sha256(blank)
+    text = (
+        f"![Figur](/assets/{after[:12]}-figur.png)\n"
+        f"Image: figur.bmp (8x4 px) -- converted from image/bmp sha256:{before} "
+        f"to image/png sha256:{after}\n"
+    )
+    build = _build(assets={f"{after[:12]}-figur.png": after}, bundle_text=text)
+    assert gate.asset_holds(build, source) is True
+
+    doc = gate.asset_holds.__doc__ or ""
+    lowered = doc.lower()
+    assert "fidelity" in lowered, "the judge does not say what its second route cannot see"
+    assert "pixel" in lowered, "the limit is stated without naming what is not checked"
+
+
 def _declared(heading: int = 2, image: int = 1, fate: str = "rejected") -> dict[str, Any]:
     return {
         "accounting_version": 1,