v2k: support rbd cutover via mapped block devices

[dhslove]

Summary

• support v2k cutover for rbd targets using host-side mapped block devices
• keep sync path on direct rbd: targets and use mapped /dev/rbd/<pool>/<image> only for bootstrap/cutover/libvirt
• persist mapped RBD device state in manifest runtime and remap after WinPE shutdown
• improve fleet sync status so rbd migrations report used bytes via rbd du
• isolate linux bootstrap NBD cleanup to reduce cross-VM interference
• update v2k docs, test runbook, CI workflow encoding, and repo rules for binary artifacts

Details

• rbd cutover now defines libvirt disks as block devices
• Linux bootstrap supports mapped rbd roots
• WinPE bootstrap/virtio detection and failure flow were hardened
• final Windows boot remaps RBD devices after WinPE shutdown
• fleet status now shows transferred size for rbd targets

Validation

• Windows cutover completed successfully with rbd
• WinPE stage completed successfully with test WinPE ISO
• Linux bootstrap/cutover issues were mitigated with tighter NBD isolation
• rbd base sync progress now reports used bytes