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/architecture/schapi.md

@@ -1,4 +1,4 @@
-# SchAPI, and Schunix as a system
+# SchAPI, and SchOS as a system
 
 Status: **direction agreed, not yet implemented.** Nothing in this document
 changes what is being built this week; it changes what everything is being
@@ -8,7 +8,7 @@ built *towards*. See §7 for what actually happens now.
 
 ## 1. The decision
 
-Schunix stops being "a Unix-like kernel with a userland bolted on" and becomes
+SchOS stops being "a Unix-like kernel with a userland bolted on" and becomes
 a **system**: a kernel, a native API, and the services and applications that
 sit on it, designed together.
 
@@ -50,7 +50,7 @@ Linux made the opposite choice — the syscall ABI *is* the contract, "we do not
 break userspace" — and consequently every syscall mistake ever made is still
 there, with a `2` or a `64` suffixed to the name of its replacement.
 
-So, for Schunix:
+So, for SchOS:
 
 ```
     ┌──────────────────────────────────────────────┐
@@ -58,7 +58,7 @@ So, for Schunix:
     ├──────────────────────────────────────────────┤
     │  SchAPI          │  POSIX / Linux personality│   ← THE STABLE BOUNDARY
     ├──────────────────┴───────────────────────────┤
-    │  libschunix — the only thing that may syscall│
+    │  libschos — the only thing that may syscall│
     ├──────────────────────────────────────────────┤
     │  syscall table — PRIVATE, unstable, renumberable
     ├──────────────────────────────────────────────┤
@@ -69,11 +69,11 @@ So, for Schunix:
 Rules that follow, and they are not negotiable if the promise is to hold:
 
 1. **A program that issues `syscall` directly is unsupported.** It may break at
-   any release. Only `libschunix` may.
-2. `libschunix` exports versioned symbols. An entry point's behaviour never
+   any release. Only `libschos` may.
+2. `libschos` exports versioned symbols. An entry point's behaviour never
    changes; a changed behaviour is a new version of the symbol.
 3. The kernel may renumber, merge, split or delete syscalls at will, provided
-   `libschunix` still presents the same surface.
+   `libschos` still presents the same surface.
 
 This *reconciles* with the existing decision to adopt Linux syscall numbering
 (`docs/roadmap.md`, and the reason musl/toybox/mksh work at all). Linux
@@ -98,7 +98,7 @@ Every public interface carries exactly one tier:
 | `provisional` | Until the next release | Freely |
 | `internal` | None | Freely, without notice |
 
-**Everything in Schunix today is `provisional`.** The freeze starts at 1.0 and
+**Everything in SchOS today is `provisional`.** The freeze starts at 1.0 and
 covers only what is marked `stable` at that moment. This is not a way of
 dodging the commitment — it is the only way to make it, because a promise made
 now would permanently enshrine, among other things, a `fcntl(F_SETFL)` that
@@ -164,7 +164,7 @@ reasons that a single tree cannot give:
 That table is the same lesson §1 of the roadmap already learned the hard way
 with FreeBSD's `sys/dev/usb`: 100k lines welded to newbus, where porting the
 USB stack means porting the device framework first. Networking is the same
-shape. Read FreeBSD, lift NetBSD, write Schunix.
+shape. Read FreeBSD, lift NetBSD, write SchOS.
 
 ### Licensing — read this before copying anything
 
@@ -181,7 +181,7 @@ Ranked by value, not by size:
 1. **`sys/net`, `sys/netinet` — the TCP/IP stack.** The largest single win
    available. Phase 2 of the roadmap wants ARP/IPv4/ICMP/UDP/TCP, and this is a
    mature, clean, readable, 2-clause-heavy implementation of exactly that.
-2. **`sys/kern/subr_pool.c` — the pool/slab allocator.** Schunix has `kalloc`
+2. **`sys/kern/subr_pool.c` — the pool/slab allocator.** SchOS has `kalloc`
    and a page at a time. Every subsystem written since has invented its own
    fixed-size table to work around that.
 3. **`autoconf` — the driver framework** (`config(1)`, `cfdriver`/`cfattach`,
@@ -203,7 +203,7 @@ relicensed-in-notice, and maintained forever under the §3 promise.
 Not yet designed. What is decided about it:
 
 - It is the native interface; POSIX is available beneath it but not the idiom.
-- It is reached through `libschunix`, never by direct syscall.
+- It is reached through `libschos`, never by direct syscall.
 - It is designed *with* Schcom in mind, because a compositor and its clients
   are the first real users and their needs (IPC, shared buffers, event streams,
   object lifetimes) are what will shape it.
@@ -234,6 +234,6 @@ What this document changes *today* is only this:
    something to work with at 1.0.
 3. When the network stack is written, NetBSD's is the starting point rather
    than a blank file — with the licence check in §5 applied per file.
-4. `libschunix` is created when there is a second personality to justify it.
+4. `libschos` is created when there is a second personality to justify it.
    Until then, the *rule* stands even though the library does not exist yet:
    no new first-party program issues raw syscalls.