Shape text with HarfBuzz, so a font's programming ligatures draw across their cells

The SDL shell rasterized every cell by its own codepoint, so a font's `->`,
`!=` or `<=` ligature never appeared. Text is now shaped with HarfBuzz 11.0.0,
built from source like FreeType: a lazy build.zig.zon dependency (the tarball
Ghostty pins) compiled only for the gui shell, its FreeType integration
linked against the freetype package, so the binary carries no system
HarfBuzz or FreeType and a tty build never compiles it.

Shaping turns on only the font's ligature features (liga, calt, clig, rlig,
rclt); composition, local forms and fractions stay off, so a cell draws either
its nominal glyph, as it always did, or part of a ligature. A glyph no lookup
of those features covers cannot change, so its cell is resolved alone, and
that resolution is cached by codepoint: in a font without ligatures (Adwaita
Mono) no cell is ever shaped, and the pipeline draws pixel for pixel as
before.

A cell whose glyph a ligature could replace is shaped with its word: the
cells between spaces that share a decoration and a role. A word ends either
side of the cursor, block or bar, so the cell being edited shows its own
character (Ghostty's rule), and inside f|i, f|l and s|t, whose typographic
ligatures do not belong on a grid (Ghostty again). Colours never end a word:
a ligature spans a syntax colour change or a selection edge. A codepoint the
primary font lacks shapes in the fallback face that has it. Taglines are not
shaped: they are drawn at two pitches, and a strip would not line up in one.

HarfBuzz only chooses glyphs; the grid places them. A substituted glyph whose
ink reaches over neighbouring cells that draw nothing of their own (the
spacer glyphs of Fira Code-style fonts such as Maple Mono, or the cells a
merged ligature cluster swallowed) claims them: it is rasterized once as a
strip that many cells wide, and each cell samples its own slice, so
selection, the cursor and per-cell colours work unchanged. Anything the grid
cannot place (several glyphs in one cell, a positioned mark) keeps its
nominal glyphs.

A frame must not pay for this. A shaped word's atlas slots are cached by its
text (checked against the stored text, not just its hash), so a frame costs
one lookup per word and a compare for the rest of the word being walked.
The atlas is keyed by (face, glyph, role, decoration, lead, span), packed into
64 bits so a lookup hashes one word; ASCII glyph ids are a table, and a
face's HarfBuzz state is made when it is first asked about a glyph. Measured
with the latency trace, whose F line now also carries the submit time (CPU =
submit - present entry), in 10 interleaved rounds against main with an A/A
pair, at 160x50 cells: with Adwaita Mono every scenario (idle, scrolling,
typing, terminal output) is as fast or 1-3% faster, and the first frame and
time to first paint are unchanged. With Maple Mono, ligatures cost the first
frame about 0.8 ms (HarfBuzz setup and shaping the screen) and scrolling new
text about 1-2%.

With it the codepoint raster path is gone: grips look up their glyph and
stay centered, since a grip is not text. ui_font_scale_for_height, an
identity kept as a "size token", is removed (px is the size everywhere),
ui_font_has_glyph becomes ui_font_glyph, and the space slot is simply the
first, cleared cell of the atlas rather than a rasterized space. CellStyle.ul
gets an explicit u3 tag so a decoration fits in the packed keys.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

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