Replace loop device attachment with FUSE partition mounting to allow GRUB installation in container environments without privileged operations. Changes: - Add grub-probe-stub.sh that returns fake GRUB device information - Modify setup_grub() to use FUSE mounts instead of loop devices - Add helper functions for partition detection and FAT role checking - Add comprehensive unit tests for the new FUSE mount approach This enables GRUB installation on raw image files inside containers using only FUSE mounts and the grub-probe shim, eliminating the need for loop devices or other privileged operations. Describe your changes. --- - [ ] I've followed the [contribution guidelines](https://github.com/canonical/imagecraft/blob/main/CONTRIBUTING.md). - [ ] I've signed the [CLA](http://www.ubuntu.com/legal/contributors/). - [ ] I've successfully run `make lint && make test`. - [ ] I've added or updated any relevant documentation. - [ ] In documents I changed, I [added a meta description](https://canonical-starflow.readthedocs-hosted.com/how-to/add-a-page-meta-description/) if one was missing.