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

kernel/net/lwip/netif_eth.c

@@ -7,18 +7,18 @@
 // driver, and this file knows only that a network device can transmit a frame,
 // receive a frame and state its MAC address. It was `netif_e1000.c` and called
 // `e1000_transmit` directly, which meant the second NIC driver would have
-// arrived with a second near-identical copy of this file. See include/netdev.h.
+// arrived with a second near-identical copy of this file. See include/dev/netdev.h.
 //
 // One netif per bound interface, up to NETIF_MAX. Two NICs is an ordinary
 // machine -- a laptop with Ethernet and the virtual one QEMU adds is already
 // two -- and the limit exists only because a fixed array is simpler than
 // allocation for something enumerated once at boot.
 
-#include "types.h"
-#include "defs.h"
-#include "param.h"
-#include "device.h"
-#include "netdev.h"
+#include "core/types.h"
+#include "core/defs.h"
+#include "core/param.h"
+#include "dev/device.h"
+#include "dev/netdev.h"
 
 #include "lwip/opt.h"
 #include "lwip/netif.h"