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-pci-drivers.md

@@ -40,6 +40,6 @@ driver in brackets (`[-]` when unclaimed). Under QEMU's default i440fx it
 should show six devices; `tools/bootcheck.ps1` asserts on the host bridge and
 the IDE controller by ID, and on `[vga]` having bound.
 
-`inl`/`outl` were added to include/x86.h for this — config space is only
+`inl`/`outl` were added to include/arch/x86.h for this — config space is only
 reachable in dwords, so `pci_read16`/`pci_read8` shift and mask the containing
 dword rather than doing a narrower bus cycle.