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/architecture/boot.md

@@ -44,7 +44,7 @@ because there are none.** Everything the jump needs is computed before.
 ### The hand-off block
 
 One page of low memory at `0x30000`, defined once in
-[`include/schboot.h`](../../include/schboot.h) and included by both the loader
+[`include/arch/schboot.h`](../../include/schboot.h) and included by both the loader
 and `entry.S` so the two cannot disagree.
 
 | Offset | What |
@@ -463,7 +463,7 @@ what makes a loop over candidates legal at all.
 | 6 | IDE 0 | usually the ESP, so usually not SchFS |
 
 The **registered** devices are the ones found at probe time rather than known
-at build time, numbered from 4 (`include/blkdev.h`): a SATA disk behind AHCI
+at build time, numbered from 4 (`include/fs/blkdev.h`): a SATA disk behind AHCI
 (`sata0`), an NVMe namespace (`nvme0n1`), each USB stick (`usb0`..`usb3`, the
 first of them also seen as device 3), and every partition on any of them
 (`sata0p2`, `nvme0n1p2`, `usb0p2`). When a disk registers, its GPT is read, or its MBR