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

docs/architecture/graphics.md

@@ -265,7 +265,7 @@ is the mutual exclusion; the socket node never was.
 
 ## Text
 
-[`include/text.h`](../../include/text.h) + `kernel/dev/text.c`.
+[`include/gfx/text.h`](../../include/text.h) + `kernel/dev/text.c`.
 
 **The font was never the problem — the character-cell grid was.** The console
 draws text by indexing `font_data[256][16]` and stepping the cursor eight
@@ -554,7 +554,7 @@ captured framebuffer.
 
 ## One palette
 
-[`include/theme.h`](../../include/theme.h). There were three unrelated designs
+[`include/gfx/theme.h`](../../include/theme.h). There were three unrelated designs
 and the seams showed: the splash was deep purple with gold text deliberately
 matching the *panic* screen, the console was cream with near-black text, and so
 a healthy boot was dressed as a crash and then abruptly became an amber