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

etc/schinit/schdbgd.job

@@ -11,7 +11,7 @@
 # There is now a third way, and it is the one for a machine that will not boot:
 # pick "Remote debugging" in the F8 menu of a build made with DEBUG=1, and
 # schinitd starts this job despite the line below. That entry does not exist in
-# an ordinary build -- see SCHBOOT_DEBUG_FLAG in include/schboot.h.
+# an ordinary build -- see SCHBOOT_DEBUG_FLAG in include/arch/schboot.h.
 #
 # Off rather than loopback-only-and-on, even though loopback-only is already the
 # default bind, because "a listener nobody asked for" is a different kind of
@@ -20,7 +20,7 @@
 #
 # What it can reach is bounded in the kernel rather than here: schdbgd opens
 # /dev/dbgro, which refuses any view that declared itself as changing state.
-# See include/dbg.h.
+# See include/core/dbg.h.
 
 Label     = os.schos.schdbgd
 Program   = /bin/schdbgd