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/kernel-history.md

@@ -77,7 +77,7 @@ Bringing in toybox (see "Coreutils" below) surfaced four more, all fixed:
   all (`kernel/sys/linux_abi.c`) — every external command spawn from a
   program that prefers `vfork()` over `fork()` failed outright. Mapped to
   the existing `fork()` (a fully correct, if slightly slower, substitute).
-- `sys_stat`/`fstat`/`fstatat` (`kernel/sys/sys_fs.c`) filled in Schunix's
+- `sys_stat`/`fstat`/`fstatat` (`kernel/sys/sys_fs.c`) filled in SchOS's
   own native `struct stat` layout regardless of the caller — musl/toybox
   parse a fixed Linux ABI byte layout instead, so `st_mode` was garbage and
   tools like `ls` couldn't tell files from directories, or executables from
@@ -110,7 +110,7 @@ programs, pipes, and `ls`/`mkdir`/`grep` without issue.
 
 ## 2. Naming standardization
 
-Schunix standardizes on exactly one name per role, regardless of the
+SchOS standardizes on exactly one name per role, regardless of the
 underlying implementation: the shell is always installed and exec'd as
 `/bin/sh` (currently mksh, built from `ports/mksh/`), and PID 1 is always
 `/bin/init`. `cmd/init/main.c`'s exec loop only ever tries `/bin/sh` — no
@@ -161,7 +161,7 @@ Split into two stages, the standard way real bootloaders solve this:
   compiled size, and errors out (like `tools/sign.pl` does for stage 1) if
   it ever grows past that budget instead of silently corrupting the layout.
 
-The kernel now starts at disk sector 17 instead of 1 (`schunix.img`'s `dd
+The kernel now starts at disk sector 17 instead of 1 (`schos.img`'s `dd
 ... seek=17` in the root `Makefile`, and the equivalent `+(1+STAGE2_SECTORS)`
 in `stage2main.c`'s `readseg()` — all three places that encode
 `STAGE2_SECTORS` have to agree, or the kernel won't be found on disk at
@@ -184,7 +184,7 @@ renderer in `stage2main.c` (not the kernel's `vga.c` — paging isn't set up
 yet, so the LFB physical address from the VBE block is used as a plain
 pointer) reusing the kernel's own font data (`include/font.h`) directly.
 It fills the framebuffer with PSOGM's purple (matching
-`kernel/core/console.c`'s panic screen) and draws "SCHUNIX" centered, then:
+`kernel/core/console.c`'s panic screen) and draws "SCHOS" centered, then:
 
 - **Real-time loading bar**: `readsect()` (in `stage2main.c`, not the
   kernel) calls `progress_tick()` after every 512-byte disk read, updating
@@ -210,7 +210,7 @@ It fills the framebuffer with PSOGM's purple (matching
   `sys_reboot()`); a real bootloader can't jump into BIOS/UEFI setup
   directly, so this is the closest honest equivalent — it gives the user
   a fresh POST during which their firmware's own setup-menu key works.
-- **Verbose boot**: picking "Boot Schunix (verbose)" writes a flag byte to
+- **Verbose boot**: picking "Boot SchOS (verbose)" writes a flag byte to
   `BOOT_VERBOSE_FLAG_ADDR` (`VBE_INFO_ADDR + 0x100`, i.e. just past the VBE
   block) before jumping to the kernel. `kernel/core/main.c`'s `kmain()`
   reads it into a global `boot_verbose` as its very first action and, if
@@ -496,7 +496,7 @@ became an amber terminal. `include/theme.h` is now the one palette, and the
 rule is: **normal states are calm and dark; only failure is loud.** PSOGM keeps
 the purple entirely to itself, which is what makes it mean something.
 
-The splash copy went from "Schunix Boot Manager / version 1.0 / Memory: 1023 MB
+The splash copy went from "SchOS Boot Manager / version 1.0 / Memory: 1023 MB
 / Press F8 for boot menu..." to the product name, a rule in the accent colour,
 and the one hint that is actionable — moved to the bottom, where a hint
 belongs. No shipping OS names its bootloader on the splash.