The phased plan taking the volume-manager track from one FAT volume to any
filesystem, N volumes, content identity, remount, and driver-crash survival:
S1 identity ladder (GPT GUID + FAT serial), S2 volumes.csv/filesystems.csv mount
map, S3 multi-volume, S4 exFAT (the second engine proving the harness reuse),
S5 removal robustness (medium_changed consumption + driver-crash rebuild +
QEMU-verified remount). Code-grounded: real functions, commit-granular steps,
discrimination tests shown to fail against today's behavior, per-phase risks.
Design decisions taken with recommended defaults; two forks flagged for review
(the /volumes/usb transitional naming and the exFAT write scope). Not started.
