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/fs/buf.h

@@ -0,0 +1,26 @@
+struct buf {
+  int flags;
+  uint dev;
+  // 64-bit, because the filesystem's block numbers are. This was a uint, which
+  // capped any device at 2^32 blocks (16 TB at 4 KB) no matter what the format
+  // above it could address -- SchFS is a 64-bit format and was being served by
+  // a 32-bit cache.
+  uint64 blockno;
+  struct sleeplock lock;
+  uint refcnt;
+  struct buf *prev; // LRU cache list
+  struct buf *next;
+  struct buf *qnext; // disk queue
+  struct buf *hnext; // hash chain, by (dev, blockno)
+  struct buf *anext; // every buffer, in the order made; never reordered
+  // BSIZE bytes, a page of its own. A pointer rather than an array since the
+  // cache grows at run time (see bio.c); drivers copy through their own DMA
+  // buffers, so where the page sits does not matter to them.
+  uchar *data;
+};
+#define B_VALID 0x2  // buffer has been read from disk
+#define B_DIRTY 0x4  // buffer needs to be written to disk
+
+
+// How many buffers the cache holds now, and the most it will grow to. bio.c.
+void bcache_size(uint *n, uint *max);