install: Enable installing to multi device parents

[ckyrouac]

When the root filesystem spans multiple backing devices (e.g., LVM across multiple disks), discover all parent devices and find ESP partitions on each. For bootupd/GRUB, install the bootloader to all devices with an ESP partition, enabling boot from any disk in a multi-disk setup. systemd-boot and zipl only support single-device configurations.

This adds a new integration test validating both single-ESP and dual-ESP multi-device scenarios.

Fixes: #481

Assisted-by: Claude Code (Opus 4.5)

[gemini-code-assist]

Code Review

This pull request successfully enables installing to multi-device parent filesystems, such as LVM spanning multiple disks. It correctly discovers all parent devices and, for bootupd/GRUB, installs the bootloader to all devices with an ESP partition. For bootloaders that only support single-device configurations like systemd-boot and zipl, the implementation correctly defaults to using the first available device. The changes are well-architected, adapting data structures and logic to handle multiple devices. A new, thorough integration test validates both single and dual ESP scenarios. Overall, this is a solid enhancement with good error handling and logging. I have one suggestion to further improve the robustness of ESP detection.

[ckyrouac]

waiting to merge until the patch release goes out

[HuijingHei]

Maybe this PR could help to consider #1916 ? @ckyrouac