fat: mount every FAT32 volume under /disk/<UID>/, fix multi-entry LFN

- fat.rs: VOL: Option<Volume> → VOLS: Vec<Volume>; try_mount() returns
  a Volume per disk and init() mounts every valid FAT32 volume found.
  Volume UID = sanitized uppercase label + 8-hex volume serial
  (e.g. RSDOS-5BD1E55F): unique per format, stable across reboots,
  human-recognizable, machine-parseable (^[A-Z0-9_]+-[0-9A-F]{8}$);
  cloned volumes get a -N suffix.  Public ops take a vol index and
  select it (select_vol also picks the backing AHCI disk), matching
  the ahci::select pattern
- fs.rs: /disk is now a plain directory whose children are one Disk
  node per mounted volume, named by UID; DiskInfo carries vol
- LFN fix: entries are stored last-first on disk — both the writer
  (head-entry first) and reader (append in disk order) were reversed
  and cancelled out for ≤13-char names; multi-entry names (>13 chars)
  read back mangled (e.g. 'from-kernel.txt' as 'xt').  Writer now
  places seq=1 closest to the SFN and seq=N|0x40 first; reader places
  chunks by sequence number and trims at the first NUL.  Verified with
  a 25-char filename round trip: kernel ls/cat correct, mtools shows
  the full LFN, fsck clean on both volumes
- test-disk.sh: test file renamed to disk-test-.txt (>13 chars,
  exercises multi-entry LFN); UID discovered from the boot log; QEMU
  stdio redirected to a log file + EXIT trap so an orphaned QEMU can
  never hang a piped caller (it previously did, 400s)

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