Fix memtest ISO validation false negatives

bible-local/docs/iso-build-rules.md

@@ -32,6 +32,9 @@ contains all of the following:
 Rules:
 
 - Keep explicit post-build memtest validation in `build.sh`.
+- Treat ISO reader success as a separate prerequisite from memtest content.
+  If the reader cannot list or extract from the ISO, that is a validator
+  failure, not proof that memtest is missing.
 - If built-in integration does not produce the artifacts above, use a
   deterministic project-owned copy/extract step instead of hoping live-build
   will "start working".
@@ -53,3 +56,7 @@ Known bad loops for this repository:
   timing is late enough to patch final `binary/boot/grub/grub.cfg` or
   `binary/isolinux/live.cfg`; logs from 2026-04-01 showed those files were not
   present yet when the hook executed.
+- Do not treat a validator warning as ground truth until you have confirmed the
+  ISO reader actually succeeded. On 2026-04-01 we misdiagnosed another memtest
+  regression because the final ISO was correct but the validator produced a
+  false negative.