docs: retract "reference renderer" -- sylpheed-cli is not the oracle

A framing correction from the human, and it runs through everything I have
written, so it is a retraction rather than a silent edit.

Reborn "was/is just a GUI explorer and extraction CLI for verifying the decoding
of the various files. It may very well be wrong." The oracle is the Xenia Canary
capture and the game.

So verify-screen is a CONSISTENCY check between two decoders that share their
assumptions, plus a regression detector -- not a correctness check, and
agreement in it is not evidence of correctness. Its header now says so, it calls
the CLI the COMPARISON renderer, and DIFFERS means "we moved apart, find out
which of us moved".

The uncomfortable part, recorded because it is the actual failure mode: this
file already contained the sentence "two renderers reading one field through one
decoder agreeing is not evidence that the field is right", written after the
ptframe1 case -- and I then went on quoting 3/255 against sylpheed-cli as though
it meant the port was right. Having the principle written down did not stop me
leaning on the agreement.

Three times both renderers agreed and both were wrong, each caught only by a
capture: pteff05 (menu screens had no background), scale-0 (drawn full size
instead of collapsed), rest() (the menu bracket missing).

Correctness moves to the captures -- nine of them, indexed at
docs/re/captures/ORACLE-CAPTURES.md, covering all five screens in scope. Three
cautions travel with them: not gamma-neutral (there is a floor, don't chase it),
geometry IS sound (a positional disagreement is real), and each is one moment of
a still-animating screen.

verify-screen keeps running over all 16 screens every iteration. It is still
worth having -- total, cheap, and it catches a divergence introduced on the RE
side. It is just not a grade.

tools/verify-screen

@@ -1,6 +1,23 @@
 #!/usr/bin/env bash
 # Diff Godot's drawing of an exported screen against `sylpheed-cli screen
-# render` of the same build -- the P1 gate.
+# render` of the same build.
+#
+# WHAT THIS IS, AND WHAT IT IS NOT.
+#
+# It is a CONSISTENCY check between two decoders that share their assumptions,
+# and a REGRESSION detector: "did anything move since last commit". It is NOT a
+# correctness check and agreement here is NOT evidence of correctness.
+#
+# `sylpheed-cli` is not the oracle. The oracle is the Xenia Canary capture and
+# the game. Reborn is an explorer and extraction CLI for verifying decodes, and
+# it can be wrong -- this corpus has been bitten three times by both renderers
+# agreeing and both being wrong: pteff05 (the menu background, missing from
+# both), scale-0, and rest(). Each time the capture caught it and neither
+# renderer could have.
+#
+# So: a DIFFERS row means "we moved apart, go find out which of us moved". It
+# does not mean the port is wrong. Where a capture and this tool disagree, the
+# capture wins. Use `tools/verify-capture` for the correctness question.
 #
 #   tools/verify-screen                 # every screen in the manifest
 #   tools/verify-screen main_menu title # named screens
@@ -11,7 +28,7 @@
 #
 # The two renderers are held to the same inputs on purpose:
 #
-#  * the REFERENCE CLI is the one built by `build-reference-cli`, from the same
+#  * the COMPARISON CLI is the one built by `build-reference-cli`, from the same
 #    `sylpheed-formats` revision the exporter is pinned to. /reborn's own
 #    target/ is a live mount of the other agent's checkout and moves mid-run; a
 #    pixel disagreement against a moving decoder proves nothing.
@@ -30,8 +47,9 @@
 #    PORT against the reference. It is not the test of whether `rest` is right;
 #    that one is the oracle capture, and the port already departs from it.
 #
-# A difference here is not automatically the port's fault. Say which renderer is
-# wrong and why -- do not tune until they match.
+# A difference here is not automatically the port's fault, and it is not
+# automatically a fault at all. Say which renderer moved and why -- do not tune
+# until they match.
 set -euo pipefail
 cd "${PROJECT_DIR:-/work}"