feat: enable systemd-boot for ostree backend via BLS translation to ESP

Currently `bootc install to-disk --bootloader systemd` bails with "bootupd is required for ostree-based installs" when using the ostree backend. This blocks using systemd-boot on ostree-based systems.

## The problem

ostree needs hardlinks and symlinks on /boot for atomic deployment swaps, so it requires a POSIX filesystem (ext4/XFS). systemd-boot can only read FAT32. These are fundamentally incompatible -- ostree can't write to FAT32, systemd-boot can't read ext4.

The composefs backend avoids this entirely by not using ostree's /boot management. But for the ostree backend, there's no path forward today.

## Proposed approach: translation layer

I have a working implementation (PR incoming) that adds a translation layer between ostree's /boot and the ESP:

1. Create an ext4 /boot partition for ostree's internal use (hardlinks, symlinks, atomic swaps)
2. Set `sysroot.bootprefix=false` so BLS paths are relative to /boot root
3. After ostree writes BLS entries to /boot, copy them (plus kernel + initrd) to the FAT32 ESP

This gives ostree the POSIX filesystem it needs while giving systemd-boot the FAT32 it needs. The ESP has plenty of space (512MB) for a couple of kernels + initrds.

Tested on Fedora 42 with QEMU + OVMF 4M -- system boots with systemd-boot v257, ostree deployment active, kargs.d entries in /proc/cmdline.

## Upgrade path (needs discussion)

The install-time sync is straightforward because ostree has already written BLS entries to /boot by the time we copy them. The upgrade path is harder: `stage_tree_with_options` stages the deployment but does NOT write new BLS entries to /boot -- that happens during the next boot via `ostree-finalize-staged.service`. So there's nothing to copy to the ESP before reboot.

Options I see:

1. Generate BLS entries from staged deployment metadata and write directly to ESP
2. Hook into `ostree-finalize-staged.service` to sync ESP during early boot
3. Wait for ostreedev/ostree#1967 (renameat2 on vfat) which would let ostree manage a FAT32 /boot directly

I'd appreciate guidance on which approach fits best with bootc's architecture.

## Related

- #92 -- original systemd-boot RFE
- ostreedev/ostree#1719 -- ostree systemd-boot support discussion
- systemd/systemd#40232 -- ext4 XBOOTLDR broken in systemd-boot
- systemd/systemd#40131 -- Poettering confirms XBOOTLDR must be VFAT