feat(font): port the Linux font engine

Ports the rest of ZIG/src/font for the freetype/fontconfig/harfbuzz
backend: face loading and rasterization (synthetic bold/italic, color
emoji bitmaps, glyph constraints for nerd font icons), fontconfig
discovery, DeferredFace, Collection, CodepointResolver, CodepointMap,
SharedGrid, opentype table parsers, the nerd font attribute table, and
the harfbuzz shaper with feature parsing and shaping cache.

Embedded fonts (JetBrains Mono 2.304 variable, Symbols Nerd Font 3.4.0,
Noto emoji) and the test fonts from Ghostty live in res/ with their OFL/
MIT licenses.

The crate opts into unsafe_code = "deny"; FFI is confined to ft.rs,
fontconfig.rs and hb.rs with SAFETY comments. Glyph resampling uses an
stb-style resampler that is not bit-identical to stb.

Deferred to the next font pass (needs ghostty-terminal/config):
shaper/run.zig, SharedGridSet.zig and the 23 terminal-driven shaper
tests. Config-facing types are mirrored locally in config_types.rs.

Test Plan:
- cargo test -p ghostty-font (139 tests)

crates/ghostty-font/src/sprite/face.rs

@@ -26,22 +26,8 @@ use crate::atlas::{self, Atlas};
 use crate::glyph::Glyph;
 use crate::metrics::Metrics;
 
-/// Text/emoji presentation.
-// TODO(integration): replace with the real `font::Presentation` once the
-// rest of the font engine (face.zig / main.zig) is ported.
-#[derive(Debug, Clone, Copy, PartialEq, Eq)]
-pub enum Presentation {
-    Text,
-    Emoji,
-}
-
-/// The subset of `font.face.RenderOptions` that the sprite face uses.
-// TODO(integration): replace with the real `font::face::RenderOptions`.
-#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
-pub struct RenderOptions {
-    /// The number of grid cells this glyph will take up (0..=3, a `u2` in Zig).
-    pub cell_width: Option<u8>,
-}
+pub use crate::Presentation;
+pub use crate::face::RenderOptions;
 
 /// Errors from [`Face::render_glyph`].
 #[derive(Debug, Clone, PartialEq, Eq, thiserror::Error)]
@@ -469,7 +455,14 @@ mod tests {
             for cp in cps {
                 for cell_width in [None, Some(1), Some(2)] {
                     let g = face
-                        .render_glyph(&mut atlas, cp, RenderOptions { cell_width })
+                        .render_glyph(
+                            &mut atlas,
+                            cp,
+                            RenderOptions {
+                                cell_width,
+                                ..RenderOptions::new(metrics)
+                            },
+                        )
                         .unwrap_or_else(|e| panic!("cp {cp:X} {w}x{}: {e}", asc + desc));
                     // The glyph must stay within the padded canvas.
                     let cw = metrics.cell_width * u32::from(cell_width.unwrap_or(1).max(1));
@@ -538,7 +531,7 @@ mod tests {
             .render_glyph(
                 &mut atlas,
                 Sprite::CursorBar as u32,
-                RenderOptions::default(),
+                RenderOptions::new(face.metrics),
             )
             .unwrap();
         // 1px bar placed at x = -1 (half its thickness rounded up).