include: one folder per subsystem, and includes that name it

Sixty-five headers sat side by side in include/, so `#include "irq.h"`
said nothing about where irq.h was or what it belonged to, and errno.h,
stat.h and fcntl.h at the top shadowed the system's for any host tool
built with -I./include. They are in folders now -- arch/ core/ fs/
dev/ gfx/ net/ uapi/ (and ldls/ for the Linux driver layer) -- and
every include names the folder, as Linux's do: "core/proc.h".

Done mechanically and checked the hard way: the kernel built from this
tree disassembles identically to the one built from the tree before
it. The full image boots, and userland is the same set of programs.
Paths in docs and comments follow; docs/guides/layout.md has a table
of what is where, and tools/fix-includes.py rewrites old-style
includes from the folders in the tree, for code written before this.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S1giNvrHTrPMPw7V6x1kQ4

docs/Website/Assets/uberblock.svg

@@ -17,7 +17,7 @@
   It is marked twice over: it carries the accent, the same colour as the root at
   the centre, and it reaches outward past the ring while sharing the inner edge
   with every other slot. The colour is the reading on screen; the size is what
-  survives include/logo.h, which is a single coverage channel and keeps no
+  survives include/gfx/logo.h, which is a single coverage channel and keeps no
   colour at all.
 
   Stands alone, with no reference to SchOS, as projects/schfs-linux/README.md