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-device-model.md

@@ -8,7 +8,7 @@ metadata:
   modified: 2026-08-08T18:05:13.551Z
 ---
 
-Built 2026-08-08. `include/device.h` + `kernel/dev/device.c`, readable at
+Built 2026-08-08. `include/dev/device.h` + `kernel/dev/device.c`, readable at
 `/proc/devices`. Written up in `docs/architecture/devices.md`. Supersedes the two separate
 arrays in `kernel/drivers/registry.c` as the answer to "what is attached".
 
@@ -35,7 +35,7 @@ break both assumptions** and needs a spinlock plus a reader lock in procfs.
 
 ## The display class
 
-`include/display.h` + ops in `kernel/dev/vga.c`, published on the device by
+`include/gfx/display.h` + ops in `kernel/dev/vga.c`, published on the device by
 `vga_pci_driver.ops`. `struct pci_driver` gained an `ops` field for this —
 **it has to live on the driver, not be set inside probe()**, because
 `pci_bind()` registers the device *after* probe returns.