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, 1 insertions, 3 deletions

diff --git a/rsnix/src/vt.rs b/rsnix/src/vt.rs
index 88129f2..59378f3 100644
--- a/rsnix/src/vt.rs
+++ b/rsnix/src/vt.rs
@@ -432,9 +432,7 @@ fn scroll_up() {
         // appears promptly.
         const TICK_LIMIT: u16 = 4;
         let now = crate::interrupts::TICKS.load(core::sync::atomic::Ordering::Relaxed);
-        if PENDING_SCROLLS >= SCROLL_BATCH
-            || now.wrapping_sub(LAST_SYNC_TICK) > TICK_LIMIT
-        {
+        if PENDING_SCROLLS >= SCROLL_BATCH || now.wrapping_sub(LAST_SYNC_TICK) > TICK_LIMIT {
             sync_scroll();
         }
     }