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/licensing.md

@@ -1,13 +1,13 @@
 # Licensing
 
-The one page to check before copying anyone else's code into Schunix.
+The one page to check before copying anyone else's code into SchOS.
 
 Referenced from `LICENSE`, `LICENSE.MIT` and `NOTICE`, all three of which
 pointed here long before this file existed.
 
 ---
 
-## 1. Schunix is Apache-2.0
+## 1. SchOS is Apache-2.0
 
 Copyright Nexoniarz. See `LICENSE`.
 
@@ -19,7 +19,7 @@ outside contributions, that silence is the expensive kind.
 
 What that grant means in practice: Apache-2.0 §2 gives everyone a perpetual,
 worldwide, irrevocable licence to reproduce, modify, sublicense and distribute
-Schunix. Any additional restriction stated elsewhere — in a README, a website,
+SchOS. Any additional restriction stated elsewhere — in a README, a website,
 or a policy — does not narrow it. The licence text is what holds up.
 
 If restricted distribution is ever genuinely wanted, that is a licence change
@@ -57,7 +57,7 @@ licences at once.
 
 GPL-3.0 fixed this in the other direction (Apache-2.0 is compatible *into*
 GPLv3), but that does not help: the result would have to ship under GPLv3, and
-Schunix does not.
+SchOS does not.
 
 **Practical consequence:** BSD drivers can be adopted. Linux drivers cannot,
 ever, no matter how convenient. This is why the USB stack was written from the
@@ -113,7 +113,7 @@ sys/netinet6    51 files,  5   "        "         "
 ```
 
 Encouraging at first glance — the 25 in `sys/netinet` are almost all SCTP,
-DCCP and multicast routing, none of which Schunix needs. `ip_input.c`,
+DCCP and multicast routing, none of which SchOS needs. `ip_input.c`,
 `ip_output.c`, `udp_usrreq.c`, `if_arp.c`, `ip_icmp.c`, `if.c` and `route.c`
 are all UC 3-clause, i.e. fine.
 
@@ -161,7 +161,7 @@ tables and FreeBSD's locking. Taking it means porting FreeBSD's kernel
 infrastructure first and finding the TCP somewhere underneath, which is the same
 conclusion `docs/roadmap.md` §1 already reached about `sys/dev/usb`.
 
-lwIP was written for the case Schunix is in: a small operating system with a raw
+lwIP was written for the case SchOS is in: a small operating system with a raw
 Ethernet driver and no infrastructure to speak of. Its port layer is a handful
 of files, it has no dependency on the host kernel's allocator or locking, and it
 brings ARP, IPv4, ICMP, UDP, TCP, DHCP and DNS.
@@ -175,7 +175,7 @@ Chosen 2026-08-09, both licences checked at source rather than assumed:
 | Roboto | desktop, window manager, compositor | **Apache-2.0** |
 | JetBrains Mono | terminal, TTY | **SIL OFL 1.1** |
 
-Roboto is the same licence as Schunix, so it is the easy case: a NOTICE entry
+Roboto is the same licence as SchOS, so it is the easy case: a NOTICE entry
 and nothing else.
 
 OFL is permissive and bundling is precisely what it is for, with two conditions
@@ -195,7 +195,7 @@ worth knowing before anyone subsets or converts a font:
 ## 4. Things that are deliberately not shipped
 
 Some code is fine to *build and run locally* and impossible to *distribute*
-with Schunix. Keeping the distinction sharp is what stops it eroding:
+with SchOS. Keeping the distinction sharp is what stops it eroding:
 
 - **`Refrence/`** — a FreeBSD checkout (`Refrence/freebsd-src`), kept as
   reference material and a parts bin. Never built, never in an image. The