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/main.c

@@ -1,19 +1,19 @@
-#include "types.h"
-#include "defs.h"
-#include "vt.h"
-#include "gfx.h"
-#include "param.h"
-#include "memlayout.h"
-#include "mmu.h"
-#include "proc.h"
-#include "x86.h"
-#include "irq.h"
-#include "dma.h"
-#include "klog.h"
-#include "usb.h"
-#include "acpi.h"
-#include "vmap.h"
-#include "hwinfo.h"
+#include "core/types.h"
+#include "core/defs.h"
+#include "dev/vt.h"
+#include "gfx/gfx.h"
+#include "core/param.h"
+#include "arch/memlayout.h"
+#include "arch/mmu.h"
+#include "core/proc.h"
+#include "arch/x86.h"
+#include "core/irq.h"
+#include "core/dma.h"
+#include "core/klog.h"
+#include "dev/usb.h"
+#include "dev/acpi.h"
+#include "arch/vmap.h"
+#include "core/hwinfo.h"
 
 static void *klog_base;   // the persistent log, just above the kernel image
 
@@ -37,7 +37,7 @@ int boot_debug;
 // Must match SCHBOOT_VERBOSE_FLAG in boot/bios/stage2main.c exactly: just past
 // the VBE info block, not 0x7000 (startothers() below uses that address for
 // the AP boot trampoline).
-#include "schboot.h"
+#include "arch/schboot.h"
 
 // Bootstrap processor starts running C code here.
 // Allocate a real stack and switch to it, first
@@ -174,7 +174,7 @@ kmain(uint32 fb_addr)
   // kalloc's free list makes no such promise -- two pages from it are adjacent
   // only by chance. Taking it first is the only way to be sure the region
   // exists; asking for it later would mean asking kalloc for something it
-  // fundamentally cannot provide. See include/dma.h.
+  // fundamentally cannot provide. See include/core/dma.h.
   // Before anything touches PHYSTOP, which is now a variable read from the
   // firmware's memory map rather than a constant asserting 224 MB exists.
   e820_init();
@@ -295,7 +295,7 @@ kmain(uint32 fb_addr)
   //
   // main() names no display driver. Whichever one owns the boot framebuffer
   // publishes it there as this kernel's scanout, and the console and crash
-  // screen draw on that; see include/gfx.h.
+  // screen draw on that; see include/gfx/gfx.h.
   //
   // No splash drawing here on purpose. The loader owns the splash, logo and
   // all, and bringing the framebuffer up deliberately does not clear it -- so
@@ -357,7 +357,7 @@ kmain(uint32 fb_addr)
   //    every entry from user mode, so both CPUs shared one per-CPU block.
   // 4. `cpu` and `proc` were __thread. That means per *thread*, so the compiler
   //    cached the thread pointer across calls -- but %fs here is per *CPU*, and
-  //    a kernel thread changes CPU at every sched(). See include/proc.h.
+  //    a kernel thread changes CPU at every sched(). See include/core/proc.h.
   // 5. The new per-CPU slots collided with syscall_entry's scratch word at
   //    %fs:(8), which is the same memory as %gs:8.
   //