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/dev/keymap.h

@@ -0,0 +1,50 @@
+// Keyboard layouts.
+//
+// A layout answers one question: given a scancode and the modifiers held,
+// which Unicode codepoint was typed? Everything else about the keyboard --
+// which port to read, when a key is a release, what the terminal-switch chord
+// is -- belongs to kernel/dev/kbd.c and is the same whatever the layout.
+//
+// Codepoints, not bytes. A layout that can only produce bytes can produce
+// `ó` (U+00F3, which fits in Latin-1) but not `ą` (U+0105, which does not),
+// and half a Polish alphabet is not a Polish layout. kbd.c encodes the
+// codepoint to UTF-8 on the way to the console.
+//
+// **No dead keys.** On a German layout the acute and circumflex keys are dead
+// -- pressed, they wait for the next key and combine -- and here they simply
+// produce the accent character on its own. That is wrong, it is visibly
+// wrong, and it needs somewhere to hold "a dead key is pending", which is
+// per-keyboard state this interface has no room for. Layouts that need it are
+// not shipped rather than shipped broken; see keymap_layouts[] for which.
+
+#ifndef KEYMAP_H
+#define KEYMAP_H
+
+#include "core/types.h"
+
+// Which of a layout's four tables a set of modifiers selects. Plain, Shift,
+// AltGr, and Shift+AltGr -- Ctrl is not one of them, because a control
+// character is derived from the plain codepoint rather than looked up, which
+// is what makes Ctrl+Z still be Ctrl+Z on a layout that moved the Z key.
+#define KM_PLAIN        0
+#define KM_SHIFT        1
+#define KM_ALTGR        2
+#define KM_SHIFTALTGR   3
+#define KM_LEVELS       4
+
+struct keylayout {
+  const char   *name;                   // "us", "pl", "de" -- what you type
+  const char   *desc;                   // what it is, for `loadkeys -l`
+  const uint32 *level[KM_LEVELS];       // 256 entries each; NULL falls back
+};
+
+// The active layout, and the catalogue.
+const struct keylayout *keymap_active(void);
+const struct keylayout *keymap_by_index(int i);   // NULL past the end
+int  keymap_set(const char *name);                // 0, or -1 if unknown
+
+// Scancode plus modifier bits (SHIFT/CTL/ALT/RALT/CAPSLOCK from kbd.h) to a
+// Unicode codepoint, or one of the KBD_* specials, or 0 for "no character".
+uint32 keymap_lookup(uint scancode, uint mods);
+
+#endif /* KEYMAP_H */