fix(iso): make memtest non-blocking by default

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

@@ -17,6 +17,25 @@ This applies to:
 
 ## Memtest rule
 
-Prefer live-build's built-in memtest integration over custom hooks or hardcoded
-bootloader paths. If you ever need to reference memtest files manually, verify
-the exact package file list first for the target Debian release.
+Do not assume live-build's built-in memtest integration is sufficient for `bee`.
+We already tried that path and regressed again on 2026-04-01: `lb binary_memtest`
+ran, but the final ISO still lacked memtest binaries and menu entries.
+
+For this project, memtest is accepted only when the produced ISO actually
+contains all of the following:
+
+- `boot/memtest86+x64.bin`
+- `boot/memtest86+x64.efi`
+- a memtest entry in `boot/grub/grub.cfg`
+- a memtest entry in `isolinux/live.cfg`
+
+Rules:
+
+- Keep explicit post-build memtest validation in `build.sh`.
+- 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".
+- Do not switch back to built-in-only memtest without fresh build evidence from
+  a real ISO.
+- If you reference memtest files manually, verify the exact package file list
+  first for the target Debian release.