feat(font): port font metrics, atlas and the sprite font

Ports ZIG/src/font/{Metrics,Atlas,Glyph}.zig and the whole sprite font
(font/sprite/**): box drawing, block elements, braille, branch/git
graph symbols, geometric shapes, powerline, and the symbols for legacy
computing (+ supplement, incl. the octants table).

Ghostty draws sprites with z2d; we use tiny-skia for path coverage,
composited onto an alpha8 canvas, while following the Zig geometry
(positions, thickness, rounding). Comptime discovery of draw functions
is replaced by explicit, overlap-checked codepoint range tables.

Upstream reference sheets are vendored in testdata/ and compared with a
tolerance: braille and U+1CD00 are pixel-identical; other sheets differ
only in antialiasing on thin diagonals/curves. Review renders are written
to target/sprite_sheets/.

The crate temporarily drops its config/unicode/terminal deps until the
rest of the font engine is ported (Phase 3).

Test Plan:
- cargo test -p ghostty-font (49 tests incl. reference sheet comparison)
- inspect target/sprite_sheets/review-all-18x36.png

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

@@ -0,0 +1,72 @@
+//! The built-in "sprite font": box drawing, block elements, braille, powerline
+//! and other glyphs rasterized by code, plus special sprites (underlines,
+//! cursors). Port of `font/sprite.zig`.
+
+pub mod canvas;
+pub mod draw;
+pub mod face;
+
+pub use canvas::{Box, Canvas, Color, Point};
+pub use face::Face;
+
+/// Sprites are represented as special codepoints outside of the Unicode
+/// codepoint range. Unicode maxes out at U+10FFFF (21 bits), and we use the
+/// high 11 bits to hide our special characters.
+///
+/// These characters are ONLY used for rendering and NEVER used written to
+/// text files or any other exported format, so we don't use the Private Use
+/// Area of Unicode.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+#[repr(u32)]
+pub enum Sprite {
+    Underline = Sprite::START,
+    UnderlineDouble,
+    UnderlineDotted,
+    UnderlineDashed,
+    UnderlineCurly,
+
+    Strikethrough,
+
+    Overline,
+
+    CursorRect,
+    CursorHollowRect,
+    CursorBar,
+    CursorUnderline,
+}
+
+impl Sprite {
+    /// Start 1 above the maximum Unicode codepoint.
+    pub const START: u32 = 0x1F_FFFF + 1;
+    pub const END: u32 = u32::MAX;
+
+    pub const ALL: [Sprite; 11] = [
+        Sprite::Underline,
+        Sprite::UnderlineDouble,
+        Sprite::UnderlineDotted,
+        Sprite::UnderlineDashed,
+        Sprite::UnderlineCurly,
+        Sprite::Strikethrough,
+        Sprite::Overline,
+        Sprite::CursorRect,
+        Sprite::CursorHollowRect,
+        Sprite::CursorBar,
+        Sprite::CursorUnderline,
+    ];
+
+    /// Convert a codepoint in the sprite range to a `Sprite`.
+    pub fn from_u32(cp: u32) -> Option<Sprite> {
+        Sprite::ALL.iter().copied().find(|s| *s as u32 == cp)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn sprite_start() {
+        assert_eq!(Sprite::START, Sprite::Underline as u32);
+        assert_eq!(Sprite::START + 10, Sprite::CursorUnderline as u32);
+    }
+}