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)

1 files changed, 21 insertions, 13 deletions

diff --git a/CONTEXT.md b/CONTEXT.md
index 39ce0bd..0d43b9f 100644
--- a/CONTEXT.md
+++ b/CONTEXT.md
@@ -168,19 +168,27 @@ Single Rust crate (`x86_64-unknown-uefi` target).  Module breakdown:
   geometry.  Verified in QEMU: edit/type/navigate/insert/save/exit, exact
   bytes via `cat`/`hexdump`, syntax colours via framebuffer dump.
 ✅ **AHCI + FAT32 (persistent disk)** — a SATA AHCI driver (polling DMA,
-  no interrupts) plus a FAT32 filesystem, mounted as `/disk`; every shell
-  command works on disk paths: `ls`/`cat`/`hexdump`/`echo >`/`>>`/
-  `mkdir`/`touch`/`rm`/`edit`.  The driver scans **all** implemented
-  ports (CAP.NP/PI) and starts every port with a device; `fat::init`
-  tries each disk in port order and mounts the first valid FAT32
-  volume — so the kernel finds the disk no matter which AHCI port the
-  firmware/QEMU put it on (real hardware included).  Whole-FAT cache with write-back to both
-  FAT copies, checksum-validated LFN read/write, multi-cluster files,
-  space-padded `.`/`..` dot entries (`..` → cluster 0 under the root),
-  FSInfo free-count maintenance (fsck-clean after a session, verified
-  with `fsck.fat -n`).  **Files survive reboot** — verified by
-  poweroff → `mdir`/`mtype` on the image and by a fresh boot seeing the
-  prior session's files.
+  no interrupts) plus a FAT32 filesystem, mounted under `/disk/<UID>/`;
+  every shell command works on disk paths: `ls`/`cat`/`hexdump`/
+  `echo >`/`>>`/`mkdir`/`touch`/`rm`/`edit`.  The driver scans **all**
+  implemented ports (CAP.NP/PI) and starts every port with a device;
+  `fat::init` mounts **every** valid FAT32 volume it finds, each under
+  `/disk/<UID>/` where UID = `<LABEL>-<8-hex-serial>` (e.g.
+  `RSDOS-5BD1E55F`) — sanitized uppercase label + volume serial, unique
+  per format, stable across reboots, human-recognizable and
+  machine-parseable (`^[A-Z0-9_]+-[0-9A-F]{8}$`); cloned volumes get a
+  `-N` suffix.  So the kernel finds the disks no matter which AHCI port
+  the firmware/QEMU put them on (real hardware included), and every
+  disk is visible at once (verified: `RSDOS-…` + `DATA2-…` mounted
+  side by side, writes to both, fsck-clean).  Whole-FAT cache per
+  volume with write-back to both FAT copies, checksum-validated LFN
+  read/write (multi-entry names >13 chars stored last-first on disk —
+  both the writer and reader were reversed and cancelled out; fixed),
+  multi-cluster files, space-padded `.`/`..` dot entries (`..` →
+  cluster 0 under the root), FSInfo free-count maintenance
+  (fsck-clean after a session, verified with `fsck.fat -n`).
+  **Files survive reboot** — verified by poweroff → `mdir`/`mtype` on
+  the image and by a fresh boot seeing the prior session's files.
 
   QEMU setup: `-drive id=disk,if=none,format=raw,file=disk.img` +
   `-device ide-hd,drive=disk,bus=ide.0` puts the data disk on AHCI port