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/core/uaccess.c

@@ -1,4 +1,4 @@
-// User-memory access helpers — see include/uaccess.h for the rules.
+// User-memory access helpers — see include/core/uaccess.h for the rules.
 //
 // Everything here validates against the *current* process's page table
 // (myproc()->pgdir), which is also the live CR3 while a syscall runs, so a
@@ -6,14 +6,14 @@
 // code for the remainder of the syscall (single-threaded processes can't
 // unmap it out from under us mid-call).
 
-#include "types.h"
-#include "defs.h"
-#include "param.h"
-#include "memlayout.h"
-#include "mmu.h"
-#include "proc.h"
-#include "errno.h"
-#include "uaccess.h"
+#include "core/types.h"
+#include "core/defs.h"
+#include "core/param.h"
+#include "arch/memlayout.h"
+#include "arch/mmu.h"
+#include "core/proc.h"
+#include "uapi/errno.h"
+#include "core/uaccess.h"
 
 int
 access_ok(addr_t uaddr, uint64 size)