Close the codeblocks drift: stale baselines, and cancel the GHCR plan it justified - #593

…k it justified

Re-recorded linux/ on master (dc11791, all 55) and ran local bin/dtest against
it: 55 screenshots compared, no failures. Debian ARM and Ubuntu ARM are
pixel-identical. The drift that produced five explanations over three weeks was
old amd64/Chrome-152 recordings compared against current renders.

Because a green gate and a blind gate look the same, the instrument was checked
before the result was believed: forcing background-color #ff0000 on
.post-article .highlight pre produced 16 failures - all 8 desktop plus all 8
mobile codeblocks keys and nothing else - and reverting returned it to 0. Clean
0 -> injected 16 -> reverted 0.

The consequence worth the most is a deletion. The GHCR container-publishing
plan - publish this 2.54 GB image, run CI inside it - existed solely to close a
Debian-vs-Ubuntu gap by construction. There is no gap, so it is cancelled
rather than left in the doc as a tempting someday. The rejected alternative, a
split where CI owned pixel truth and dtest was merely behavioural with
divergent keys screened, is dead too: an expected-red list is what rotted into
"everything is expected red" before #566.

What replaces both: bin/dtest is authoritative for DETECTING visual defects, so
a local red is a real defect and there is no expected-red list. CI keeps
RECORDING, because the agreement is measured rather than structural - Debian
and Ubuntu upgrade freetype independently, so it can end with nothing in this
repo changing. Reopen only on a dtest red that a master re-record then makes
green.

Two rules added, both about the shape rather than this drift:

- Suspect the baselines BEFORE the mechanism. All five theories were stories
  about a mechanism; none was a check on the instrument. "Are these current?"
  is the cheapest question available and would have ended it in a day.
- Two observers agreeing proves nothing when they share the input under test.
  Local and CI failing the same 8 keys read as corroboration and was an
  artifact - both were reading the same stale baselines, so every hypothesis
  predicted red on both.

Also records why this family and not another: dense monospace text is where any
rendering difference lands first, so the codeblocks screenshots are the suite's
canary, not an arbitrary victim - which is why they kept looking like evidence
for whichever theory was current. And docs 20.10's O3 entry is corrected; it
asserted "the 10 red Linux baselines are NOT stale", which was exactly backwards
and was repeated for three weeks.

Gates: OKF 0 ERRORs, bash -n clean, unit 289 runs / 6185 assertions / 0
failures, hugo-build green.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011SP5gaqXEgUie8pdFrmbeJ