Add critical grub2-mkconfig composefs compatibility fix

- Add comprehensive fix for grub2-mkconfig composefs incompatibility
- Document the critical issue from Red Hat Bugzilla #2308594
- Provide complete patch, automated script, and Containerfile integration
- Add troubleshooting documentation with verification steps
- Update COMPATIBILITY.md with critical issue warning
- Update README.md to highlight this critical fix

This fix is ESSENTIAL for all composefs-based bootc images to boot properly.
Without this fix, grub2-mkconfig will generate incorrect configurations
and cause boot failures.

Based on:
- Red Hat Bugzilla #2308594 (reported by Colin Walters)
- ostree issue #3198
- Affects all bootc/ostree systems using composefs

README.md

@@ -126,6 +126,9 @@ This repository contains comprehensive technical documentation for the `bootc` p
   - `base-images-wo-bootc.md` - Creating bootc images without bootc binary (Debian-specific)
   - `initramfs-integration.md` - Essential initramfs files for composefs support
 
+#### Troubleshooting
+- `troubleshooting/grub2-composefs-fix.md` - **CRITICAL**: Fix for grub2-mkconfig composefs incompatibility
+
 #### Installation
 - **`installation.md`** - Installation instructions (corrected for source compilation)
 
@@ -253,6 +256,7 @@ podman build -f examples/nginx/Containerfile -t debian-bootc-nginx:latest .
 ### Key Files for Debian Users
 - `building/base-images-wo-bootc.md` - Complete guide for creating bootc images without bootc binary
 - `building/base-images.md` - Standard methods for creating base images
+- `troubleshooting/grub2-composefs-fix.md` - **CRITICAL**: Fix for grub2-mkconfig composefs incompatibility
 - All command documentation includes Debian-specific adaptations
 
 ## Contributing