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/dev/text.c

@@ -1,19 +1,19 @@
-// Text rendering. See include/text.h for what this replaces and why.
+// Text rendering. See include/gfx/text.h for what this replaces and why.
 
-#include "types.h"
-#include "defs.h"
-#include "param.h"
-#include "text.h"
+#include "core/types.h"
+#include "core/defs.h"
+#include "core/param.h"
+#include "gfx/text.h"
 
 // Included, now that this is the only file that reads it.
 //
 // It used to be `extern unsigned char font_data[256][16];` with a note
-// explaining that include/font.h *defines* the array rather than declaring
+// explaining that include/gfx/font.h *defines* the array rather than declaring
 // it, so a second includer would break the link. The second includer was
 // gfx.c, which walked the bitmap itself when there was no TrueType face --
 // and that was the duplicate glyph path. With it gone, the header can live
 // with its one user.
-#include "font.h"
+#include "gfx/font.h"
 
 // --- UTF-8 -----------------------------------------------------------------