Rename Schunix to SchOS; the kernel becomes SchIFT

Removes the "-unix" element and with it any trademark exposure to The
Open Group.

The split follows the ecosystem layout: SchOS is the system, and the
kernel has its own name. Kernel-identity strings are the only places
that take SchIFT -- uname(2) sysname, /proc/version, and the two licence
sentences that say "kernel" outright. That is what every Unix does here:
`uname -s` on Linux reports the kernel, not the distribution. Everything
a user reads as the product -- panic screen, boot menu, error prefixes,
docs, LICENSE, NOTICE -- is SchOS.

schunix.img is schos.img, the logo assets and the musl TLS patch are
renamed, and __SCHUNIX__ is __SCHOS__ (the define and the patch that
consumes it moved together).

Two things the rename broke, both found by the bootcheck suite:

  - shmtest wrote "SCHUNIX-MMAP-OK" with a hardcoded length of 15 and
    compared it back by hand-counted offsets (m[3]=='U', m[14]=='K').
    The shorter marker left the write running two bytes past the literal
    and the compare reading past the data, failing a file-backed mmap
    that was working correctly. Now written and compared through one
    constant with sizeof.
  - bootcheck asserted a 31-byte file after appending to
    "hello-from-schunix"; the shorter string makes it 29.

Both are the same mistake: a string literal whose length is written out
somewhere else. Full suite green -- base, Tty, Ps, Sock, Fb, Usb, Evdev.

docs/drivers.md

@@ -75,9 +75,9 @@ need.
 
 ## 3. Linux drivers
 
-**Decided 2026-08-08.** Linux drivers are not shipped with Schunix and never
+**Decided 2026-08-08.** Linux drivers are not shipped with SchOS and never
 will be. The user may install one on their own machine, through a device
-manager or equivalent, and Schunix will load it.
+manager or equivalent, and SchOS will load it.
 
 The licensing reasoning matters and is worth stating exactly, because it is
 easy to get wrong in both directions: