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/fs/devfs.c

@@ -27,17 +27,17 @@
 // fileread/filewrite see T_DEV and dispatch through devsw themselves -- which
 // is why there is no read op below that does anything.
 
-#include "types.h"
-#include "defs.h"
-#include "param.h"
-#include "stat.h"
-#include "spinlock.h"
-#include "sleeplock.h"
-#include "proc.h"
-#include "fs.h"
-#include "file.h"
-#include "errno.h"
-#include "blkdev.h"
+#include "core/types.h"
+#include "core/defs.h"
+#include "core/param.h"
+#include "uapi/stat.h"
+#include "core/spinlock.h"
+#include "core/sleeplock.h"
+#include "core/proc.h"
+#include "fs/fs.h"
+#include "fs/file.h"
+#include "uapi/errno.h"
+#include "fs/blkdev.h"
 
 // Inode numbering. Two directories and a flat table of nodes, so the number is
 // either a directory id or an index into devnodes[] offset past them.
@@ -99,10 +99,10 @@ static const struct devnode devnodes[] = {
   // a flag, because the minor is the one piece of per-open state a character
   // device already sees -- so which one you opened *is* the permission, and
   // nothing has to be remembered between the write and the read. See
-  // include/dbg.h and cmd/schdbgd.
+  // include/core/dbg.h and cmd/schdbgd.
   { "dbgro",   10, 1, D_ROOT },
   { "netdev",  11, 0, D_ROOT },
-  // Audio, spelled the way UNIX spells it. See include/audio.h.
+  // Audio, spelled the way UNIX spells it. See include/dev/audio.h.
   { "dsp",     12, 0, D_ROOT },
   { "mouse",   3, 1,  D_ROOT },