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/gfx/ttf.h

@@ -0,0 +1,49 @@
+#pragma once
+
+// TrueType outlines, as a font provider for include/gfx/text.h.
+//
+// Scope, and why it is this and not "TTF and OTF":
+//
+// .ttf and .otf are both sfnt containers -- the same table directory, the same
+// cmap, the same hmtx. What differs is how outlines are stored. TrueType puts
+// quadratic B-spline contours in a `glyf` table; OpenType/PostScript puts cubic
+// béziers in a `CFF ` table encoded as Type 2 charstrings, which is a
+// stack-based interpreted format and a separate project.
+//
+// Checked before writing any of it: JetBrains Mono and Roboto -- the two fonts
+// this system is going to use -- both ship `glyf` and neither ships `CFF `. So
+// CFF support would buy nothing today, and the honest thing is to implement the
+// format the fonts actually use and say so, rather than half-implement two.
+//
+// Everything here is from Apple's TrueType Reference Manual and the OpenType
+// specification, both public documents.
+
+struct ttf {
+  const unsigned char *data;
+  uint32 len;
+
+  uint32 glyf, glyf_len;
+  uint32 loca, loca_len;
+  uint32 cmap4;               // offset of the format 4 subtable, 0 if none
+  uint32 hmtx;
+  int    num_glyphs;
+  int    num_hmetrics;
+  int    units_per_em;
+  int    long_loca;           // indexToLocFormat: 0 = 16-bit offsets, 1 = 32
+  int    ascent, descent;
+
+  // Rasterisation target size, in pixels per em. Set by ttf_open.
+  int    px;
+};
+
+// Parse a font already in memory. Returns 0, or -1 if it is not a TrueType
+// font this can read. `data` must outlive the struct -- nothing is copied.
+int ttf_open(struct ttf *, const void *data, uint32 len, int pixels_per_em);
+
+// Rasterise one codepoint into `cov`. Returns 0, or -1 if the font has no such
+// glyph or it will not fit. Exposed because the host harness uses it directly.
+int ttf_render(struct ttf *, uint32 cp, struct glyph *out, unsigned char *cov,
+               int covcap, int *ox, int *oy);
+
+// A font provider over it, for text.h. The struct must outlive the font.
+struct font *ttf_font(struct ttf *);