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

.claude/memory/schunix-linux-open-flags.md

@@ -1,6 +1,6 @@
 ---
 name: schunix-linux-open-flags
-description: "include/fcntl.h must hold Linux's open-flag values — the userland is all Linux-ABI, and a mismatch silently corrupts files"
+description: "include/uapi/fcntl.h must hold Linux's open-flag values — the userland is all Linux-ABI, and a mismatch silently corrupts files"
 metadata: 
   node_type: memory
   type: project
@@ -8,7 +8,7 @@ metadata:
   modified: 2026-08-03T13:59:19.392Z
 ---
 
-**`include/fcntl.h` holds Linux x86-64 open-flag values, and must keep holding
+**`include/uapi/fcntl.h` holds Linux x86-64 open-flag values, and must keep holding
 them.** Every program in the userland is built against a libc that sends Linux
 values; the kernel reads them with these constants. Do not "tidy" them into a
 neat sequence — they are an external interface.