ttf_words.c: TTF-TEXT entry point, hmtx advance widths (item 4.3.7e)

Punch list §25 item 4.3.7e complete. New src/word_source/ttf_words.c
registers TTF-TEXT ( c-addr u x y size color -- ): lazily loads the v1
default font capsule + raster cache once, decodes UTF-8 (C
reimplementation mirroring capsules/fabric.4th's DECODE-UTF8 exactly),
looks up each glyph's cached bitmap, blits via fb_put_pixel, advances
the pen by the glyph's real hmtx advance width scaled to pixels.

Necessary plumbing: ttf_parse() now also locates hhea/hmtx, and
ttf_glyph_advance_width() reads a glyph's advance width -- required for
this item's own "proportional spacing correct" acceptance clause, no
advance-width data existed anywhere else in the parser. Verified in
tools/ttftest.c: A/a/0/space all read advance_width=600, correctly
uniform since JetBrainsMono-Regular.ttf is monospace.

(x,y) is raster pixel space (top-left origin, Y-down), deliberately not
the stroke font TEXT's Cartesian Y-up convention -- recorded explicitly
in ttf_words.h, not conflated.

Verified live, amd64, screendump: injected
S" Hi 4.3.7e!" 200 200 28 16777215 TTF-TEXT over a serial socket after
boot, no error, captured a screendump showing the string rendered
legibly with correct mixed-case/digit/punctuation glyphs and even
spacing. TTF-TEXT is this item's permanent deliverable, not a
throwaway probe. Compile-checked clean on all three architectures.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/word_source/include/ttf_words.h

@@ -0,0 +1,38 @@
+/*
+  StarForth — Steady-State Virtual Machine Runtime
+
+  Copyright (c) 2023–2025 Robert A. James
+  All rights reserved.
+
+  Licensed under the StarForth License, Version 1.0
+*/
+
+#ifndef TTF_WORDS_H
+#define TTF_WORDS_H
+
+#include "vm.h"
+
+/**
+ * @defgroup ttf_words TrueType Text Words
+ * @{
+ *
+ * @brief `TTF-TEXT`, the TrueType-backed text entry point (FABRIC.md item
+ * 4.3.7e). Kernel-only; no-op on hosted builds (no framebuffer/capsule
+ * system exists there).
+ *
+ * @par TTF-TEXT ( c-addr u x y size color -- )
+ * Render a UTF-8 string via the v1 default font capsule
+ * (`fonts:JetBrainsMono-Regular.ttf`, loaded and cached lazily on first
+ * use). (x,y) is raster pixel space (top-left origin, Y-down) -- NOT
+ * capsules/fabric.4th's Cartesian Y-up convention used by the stroke
+ * font's `TEXT` (4.3.6f); the two are deliberately different coordinate
+ * systems, not interchangeable. (x,y) is the first glyph's baseline-left
+ * origin. `size` is capped at `TTF_CACHE_MAX_SIZE_PX` (see ttf.h).
+ * Advances proportionally per glyph via the font's real hmtx advance
+ * width, scaled to pixels.
+ * @}
+ */
+
+void register_ttf_words(VM *vm);
+
+#endif /* TTF_WORDS_H */