Explicit Bcachefs support

Currently, there is root on Bcachefs detection, but that is it...

To get proper support, we need to allow installs on Bcachefs as root (or any other mountpoint) with or without compression, encryption, multi-device support, and selecting which groups/disks to be foreground/background/promote/(metadata) targets.

There are two main blockers I can think of that need special exceptions: systemd when using a multi-device pool, and 32-bit apps (this is being worked on, and can be somewhat mitigated by forcing crc32c or another checksumming algorithm? Unsure how xxhash or chacha20-poly1305 change this!).

- systemd/systemd#8234
- koverstreet/bcachefs#650

I believe NixOS has a custom way of making systemd play nice with bcachefs involving symlinks. This is all so new and confusing to get working together, despite the theoretical benefits of an abstracted drive pool with encryption, raid, ssd cache, etc. Looks like things are coming together.

The issue names no files, tests, or entry points. Start by locating the existing root-on-Bcachefs detection and mapping the installer’s filesystem, encryption, compression, and multi-device flows. Done would require a defined scope and tests for the supported Bcachefs configurations, including the systemd and 32-bit-app constraints described here.

Written by the indexing model from the issue text.