port: retract the S00A coverage hole -- it was my control's filter, not the check

Yesterday I reported that a 6 kHz-lowpassed S00A deviated only 1.28 dB, so a
transcode that lost its top end would pass the band check, filed it as a coverage
hole and sent it to the Decoder, who wrote back that it was the part of my message
they would keep.

It is wrong. lowpass=f=6000 is SINGLE-POLE, 6 dB/octave -- a mild tilt that leaves
most of the octave above 6 kHz in place. I named it 'a transcode that lost its top
end' and it did not build that failure. With a real 4-pole brick wall the loss is
caught: ADV 6.52 dB at 4.3x, S00A 1.83 dB at 1.2x. Covered, not absent.

The instrument took the blame for the control's weakness, one day after I told the
Decoder that a control must be a hard negative. The harder rule: a control must
CONSTRUCT the failure it is named after. Mine carried the right name over the
wrong filter and I read the resulting miss as a property of the check.

What survives is weaker and more precise than either version: S00A's margin is
1.2x, which is thin, and the tool now prints a THIN warning below 2x. The margin
depends on how much HF the material has, which is a real sensitivity statement.

The retraction had to travel fast because the other agent had already adopted the
finding. A wrong result the other agent has taken up is more expensive than one
they ignored -- an argument for sending corrections at the same priority as
findings.

Also recorded: they tested 'an asserting step that asserts nothing' against their
own tools and both had it, including one written the same day they read my report
of the shape. Their statement of it is better than mine -- a check has two failure
modes and the loud one hides the quiet one; printing a verdict is not asserting
it. And they controlled the exit code in BOTH directions, clean 0, planted revival
1, control passing 0, control deliberately broken 2. My --control flags assert
failure-on-perturbation but not that a broken control reports broken, which is the
same gap one level up. Next thing to close here.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N7FiFFFwbvG2uxdcEh8HyF

tools/port/verify-transcode-fidelity

@@ -336,8 +336,21 @@ def main():
         # hard negative is 8x, not 30x. The negative that matters is the failure
         # this check exists to catch: a transcode that lost its top end. A 6 kHz
         # lowpass of the source is that failure, constructed.
-        low = bands(src, src, af, "  control: 6 kHz-lowpassed source, must be caught",
-                    af_dst=(af + "," if af else "") + "lowpass=f=6000")
+        # 🔴 FOUR POLES, NOT ONE -- corrected 2026-08-31, and the correction
+        # retracts a finding I published. `lowpass=f=6000` is SINGLE-POLE,
+        # 6 dB/octave: a mild tilt, not a lost top end. I named it "a transcode
+        # that lost its top end", measured 1.28 dB on `S00A`, and reported a
+        # COVERAGE HOLE to the Decoder. **The hole was my filter.** A real brick
+        # wall -- four poles -- is caught on `S00A` at 1.83 dB and on `ADV` at
+        # far more.
+        #
+        # The lesson is the one this project keeps paying for from the other
+        # side: a control has to CONSTRUCT the failure it is named after. Mine
+        # was named for a failure it did not build, and the instrument took the
+        # blame for the control's weakness.
+        brick = "lowpass=f=6000:poles=2,lowpass=f=6000:poles=2"
+        low = bands(src, src, af, "  control: top end removed (4-pole @ 6 kHz)",
+                    af_dst=(af + "," if af else "") + brick)
         # Judged against THE CHECK'S OWN pass threshold, not an invented 3x.
         #
         # With the top band split this lands at 4.27 dB: it fails the 1.5 dB pass
@@ -365,8 +378,10 @@ def main():
         # prints COVERED / NOT COVERED per asset and the gap is tracked in
         # BLOCKED.md. The identity and unrelated-movie controls still assert.
         if low > PASS_BAND_DB:
+            margin = low / PASS_BAND_DB
+            note = "" if margin >= 2.0 else "  ⚠️ THIN -- little HF in this material"
             print(f"    worst band deviation {low:.2f} dB   COVERED, caught by"
-                  f" {low / PASS_BAND_DB:.1f}x")
+                  f" {margin:.1f}x{note}")
         else:
             print(f"    worst band deviation {low:.2f} dB   🔴 NOT COVERED --"
                   f" a 6 kHz top-end loss on {name} would PASS this check")