Give userspace a raw disk: /dev/hda and /dev/hdb

Nothing outside the kernel could touch a disk. sys_mount does not even
look at the device it is handed -- it mounts the one SchFS volume found
at boot and ignores `source` -- so a program had no way to read a
partition table, format a volume or write a boot sector. An installer is
a program that needs all three, so this is the first piece of it.

A character device in devsw rather than a separate block layer, because
that is where the offset already lives: fileread/filewrite pass f->off
and advance it by what comes back, which is everything a seekable device
needs. The minor is the IDE device number, the same numbering iderw()
uses; two schemes for the same disks would be a bug waiting to happen.

Writes bypass every filesystem above them, which is inherent to a raw
device and why an installer must target a disk it has not mounted.

The nodes are created unconditionally, like every other entry in /dev.
That matters more than it looks: with no node, `echo x > /dev/hdb`
silently creates an ordinary file of that name and appears to work. It
did, and it agreed with itself on read-back; only the host's copy of the
disk showed nothing had been written.

st_rdev was never filled, so every node in /dev reported "0, 0" under
ls -l. Harmless while nothing read it, wrong as soon as something has to
tell two disks apart. Filled with the full Linux dev_t encoding rather
than (major << 8 | minor), which only works while both stay small.

Verified against the host as an independent oracle, both directions: the
guest wrote to /dev/hdb and the host saw those bytes at offset 0; the
host wrote at offset 4096 and the guest read them back with dd skip=.
ls -l now shows 1,1 for console, 2,0 for fb0 and 8,1 for hdb.

Known limit, commented in disk.c: devsw passes the offset as a 32-bit
uint, so a raw disk is addressable to 4 GB. That must be widened before
anyone installs onto a real disk.

bootcheck 108 [ok] with -Fb -Tty -Usb -Sock -Evdev -Ps.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Makefile

@@ -24,7 +24,7 @@ KERN_CORE = $(addprefix $(BUILD)/kernel/core/, \
               panic.o uaccess.o qr.o time.o)
 
 KERN_DEV  = $(addprefix $(BUILD)/kernel/dev/, \
-              bio.o cdev.o device.o fb.o ide.o input.o irq.o kbd.o mouse.o \
+              bio.o cdev.o device.o disk.o fb.o ide.o input.o irq.o kbd.o mouse.o \
               pci.o text.o ttf.o uart.o vga.o ramdisk.o procfs.o)
 
 # Drivers for hardware that has to be *found* rather than assumed. Anything