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

include/arch/vmap.h

@@ -0,0 +1,21 @@
+#pragma once
+
+// Kernel mappings that are neither the direct map nor plain device registers.
+// Both take one 1 GB slot of the device window ioremap() hands out, so each is
+// limited to a gigabyte and meant to be called a handful of times at boot.
+
+// Program this CPU's PAT so that a mapping can ask for write-combining. Every
+// CPU runs it before it touches a write-combined mapping -- the PAT is per
+// processor, and one left at its reset value reads the same bits as
+// write-back, which on device memory the MTRRs then turn back into uncached.
+void  pat_init(void);
+
+// Device memory, write-combined: stores are gathered into bursts rather than
+// sent one at a time, and reads are as slow as uncached. For a framebuffer,
+// never for registers. Returns 0 if there is no window left.
+void *ioremap_wc(uint64 pa, uint64 size);
+
+// `size` bytes of ordinary RAM, contiguous in the kernel's address space and
+// not physically: kalloc() pages behind a page table of their own. Zeroed.
+// For buffers too big for the DMA arena that no device reads. Never freed.
+void *vmap_alloc(uint64 size);