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/fat.h

@@ -0,0 +1,30 @@
+#pragma once
+
+// FAT12, FAT16 and FAT32, with VFAT long names: the filesystem every other
+// operating system can read, which is the whole reason to have it. A USB
+// stick, an EFI system partition, an SD card. See kernel/fs/fat/fat.c.
+//
+// Mounted the way Linux mounts it -- `mount -t vfat /dev/usb0p1 /mnt` -- and
+// "msdos" and "fat" are accepted as the same thing.
+
+// Filesystem device numbers for mounted FAT volumes. Like SCHFS_DEV, runtime
+// identities for the inode cache that nothing on disk records.
+#define FAT_VOL_SLOTS 8    // 104..111
+#define FAT_DEV       104
+#define FAT_IS_DEV(d) ((d) >= FAT_DEV && (d) < FAT_DEV + FAT_VOL_SLOTS)
+
+// The root directory's inode number on every FAT volume.
+#define FAT_ROOT_INO  1
+
+// Mount the FAT volume on block device `bdev`. Returns its filesystem device
+// number, or a negative errno: -ENODEV for no FAT there, -EBUSY for no free
+// slot or already mounted.
+int  fat_kmount(uint bdev, int readonly);
+// Flush it, mark it cleanly unmounted, and let it go.
+int  fat_kumount(uint dev);
+int  fat_readonly(uint dev);
+// Write out everything dirty on every mounted volume. `final` also marks each
+// one clean, for a shutdown that will not get to unmount them.
+void fat_ksync_all(int final);
+// Cluster size, clusters and free clusters, for statfs. -1 if not FAT.
+int  fat_kstatfs(uint dev, uint64 *bsize, uint64 *total, uint64 *freec);