storage: AHCI, GPT/MBR partitions, and a disk a PC can boot

q35 stopped with "no root filesystem" because its disk is SATA, like
every PC since 2008, and SchOS only had IDE. And SchFS could only sit
on a whole raw device, so nothing could boot from a disk that also
carried an ESP.

- include/blkdev.h, kernel/dev/blkdev.c: a registry of block devices
  found at probe time, numbered from 4. A disk's GPT (or MBR) is read
  when it registers, and each partition becomes a device whose block 0
  is the partition's first sector. SchFS needs no format change to live
  in a partition; a Linux SchFS driver will see the same bytes on
  /dev/sdX2. SchFS has its own GPT type:
  47138FA9-B79C-47A6-9F7D-AC1F7F572D5E.
- AHCI driver, written from the AHCI 1.3.1 spec: BIOS handoff,
  COMRESET when the link is down, IDENTIFY, 48-bit DMA read/write,
  FLUSH. Completion is polled, so the root disk does not depend on
  interrupt routing.
- USB sticks also register as usb0, so their partitions are found.
- The root probe tries registered devices between USB and IDE,
  SchFS-typed partitions first.
- make disk / tools/posix/mkdisk.sh: a GPT image with a 64 MB ESP and
  a SchFS partition, ready to dd to a stick or a disk.

Tested in QEMU q35: the old two-disk setup roots on sata1; disk.img as
the only SATA disk roots on sata0p2; disk.img as a USB stick (firmware
boots from it) roots on usb0p2. Writes survive sync in each case.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HH7xukt4aaiGj1nQEAoQf1

kernel/drivers/registry.c

@@ -35,6 +35,7 @@ extern const struct pci_driver e1000_driver;
 extern const struct pci_driver ac97_driver;
 extern const struct pci_driver r8169_driver;
 extern const struct pci_driver rtl8139_driver;
+extern const struct pci_driver ahci_driver;
 
 // Order matters here in one respect: virtio-gpu is offered the bus before the
 // VBE framebuffer driver, so that on a machine with both, the device with real
@@ -48,6 +49,7 @@ static const struct pci_driver *drivers[] = {
     &r8169_driver,
     &rtl8139_driver,
     &xhci_driver,
+    &ahci_driver,
     &ac97_driver,
 };