ahci: SATA driver + FAT32 persistent disk mounted at /disk

- ahci.rs: polling AHCI on PCI class 1/6/1, bus mastering + ABAR map,
  port 0 up, IDENTIFY + READ/WRITE DMA EXT through one 4KiB DMA buffer
- fat.rs: BPB parse, whole-FAT RAM cache with write-back to both copies,
  LFN read/write, dirs, multi-cluster files, FSInfo free-count sync,
  space-padded dot entries ('..' = cluster 0 under root), fsck-clean
- fs.rs: /disk mount node + 24-slot ephemeral node pool; ls/cat/hexdump/
  echo/mkdir/touch/rm/edit all work on disk paths unchanged
- QEMU: explicit bus assignment (disk=ide.0, ESP=ide.1); the implicit
  fat:rw:esp drive was stealing AHCI port 0
- Debugged against QEMU source: header DW0 is CFL in bits 4:0 / PRDTL in
  bits 31:16 (Linux libahci layout), FIS count at bytes 12-13, device at
  byte 7; QEMU rejects H2D FIS flags bits 6:4, so direction is the
  header W bit only
- Verified: full shell round-trip on /disk, reboot persistence (poweroff
  then mdir/mtype and a fresh boot), fsck.fat clean

1 files changed, 37 insertions, 4 deletions

diff --git a/CONTEXT.md b/CONTEXT.md
index 8fa4025..24a4b1e 100644
--- a/CONTEXT.md
+++ b/CONTEXT.md
@@ -97,10 +97,18 @@ Single Rust crate (`x86_64-unknown-uefi` target).  Module breakdown:
 - `paging.rs` — identity page-table helpers (map/unmap/query, large-page split)
 - `power.rs` — `ResetSystem`-based shutdown, hlt loop fallback
 - `rtc.rs` — CMOS/RTC clock (ports 0x70/0x71), BCD/12h normalization
-- `pci.rs` — legacy PCI config space (0xCF8/0xCFC), full enumeration
+- `pci.rs` — legacy PCI config space (0xCF8/0xCFC), full enumeration; `config_write`
+  - 64-bit BAR reader for the AHCI driver
+- `ahci.rs` — SATA AHCI host driver (polling, DMA, no interrupts): scans PCI for
+  class 1/subclass 6/prog-if 1, maps ABAR, starts port 0 (FRE/ST/DMAE), runs
+  IDENTIFY + READ/WRITE DMA EXT through a 4 KiB DMA buffer
+- `fat.rs` — FAT32: BPB parse, whole-FAT cache in RAM with write-back to both FAT
+  copies, cluster chains, directory scan with checksum-validated LFN
+  reassembly, LFN + 8.3 writing, FSInfo free-count maintenance, mkdir/rm/read/write
 - `fs.rs` — VFS: dirs, ramfs files, char devices (`/dev/mem`, `rtc`, `pci`,
   `version`, `meminfo`, `null`, `zero`, `console`, `serial`; `/tmp` ramfs;
-  `/etc/motd`)
+  `/etc/motd`); `/disk` mount node + 24-slot ephemeral node pool so shell
+  commands work on disk paths unchanged
 - `gop.rs` — framebuffer rasteriser (glyph blits, pixel shifts); the text
   surface itself lives in `vt`
 - `vt.rs` — terminal emulator: cell grid, cursor, ANSI CSI parser (H/J/K,
@@ -159,6 +167,23 @@ Single Rust crate (`x86_64-unknown-uefi` target).  Module breakdown:
   `tty` raw mode, stdout → `vt`, `std::fs` → VFS, SIGWINCH → fixed
   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`.  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.
+
+  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
+  0; the ESP gets `bus=ide.1`.  (The implicit `-drive file=fat:rw:esp`
+  otherwise grabs port 0.)  Make targets (`run`, `run-gui`, `debug`)
+  attach `$(DISK_IMG)` (default `/tmp/disk.img`).  OVMF writes its
+  `NvVars` varstore file onto the first FAT volume — harmless.
 
 ## Known issues / perf notes
 
@@ -181,8 +206,16 @@ Single Rust crate (`x86_64-unknown-uefi` target).  Module breakdown:
   the whole framebuffer at all.
 - **Editor redraw** re-blits changed cells per keystroke (fine at 1280×800);
   the "optimize drawing someday" item.
-- **ramfs only**: edited files live in memory, lost on reboot until disk
-  support (AHCI/FAT32) lands.
+- **AHCI quirks learned (QEMU ground truth)**: the command header layout is
+  FIS length in DW0 bits 4:0 and PRDTL in bits 31:16 (not the classic
+  CFL-at-24 bit-fields); the H2D FIS sector count lives at bytes 12-13 with
+  the device register at byte 7 (byte 10 is LBA[47:40] — no separate
+  device byte); QEMU rejects H2D FISes with flags bits 6:4 set, so the
+  write direction is signalled purely by the header W bit (1<<6).
+  Real hardware follows the same FIS layout; the header layout matches
+  Linux libahci.
+- **ramfs only for `/`**: the root filesystem is still the ramfs (lost on
+  reboot); persistent files live under `/disk` (AHCI/FAT32).
 
 ## Target audience