docs: add iso-build-rules (verify package names before use)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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

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+# ISO Build Rules
+
+## Verify package names before use
+
+ISO builds take 30–60 minutes. A wrong package name wastes an entire build cycle.
+
+**Rule: before adding any Debian package name to the ISO config, verify it exists and check its file list.**
+
+Use one of:
+- `https://packages.debian.org/bookworm/<package-name>` — existence + description
+- `https://packages.debian.org/bookworm/amd64/<package-name>/filelist` — exact files installed
+- `apt-cache show <package>` inside a Debian bookworm container
+
+This applies to:
+- `iso/builder/config/package-lists/*.list.chroot`
+- Any package referenced in `grub.cfg`, hooks, or overlay scripts (e.g. file paths like `/boot/memtest86+x64.bin`)
+
+## Example of what goes wrong without this
+
+`memtest86+` in Debian bookworm installs `/boot/memtest86+x64.bin`, not `/boot/memtest86+.bin`.
+Guessing the filename caused a broken GRUB entry that only surfaced at boot time, after a full rebuild.