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/glyph.rs

@@ -0,0 +1,21 @@
+//! Glyph is a single loaded glyph for a face. Port of `font/Glyph.zig`.
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
+pub struct Glyph {
+    /// width of glyph in pixels
+    pub width: u32,
+
+    /// height of glyph in pixels
+    pub height: u32,
+
+    /// left bearing
+    pub offset_x: i32,
+
+    /// top bearing
+    pub offset_y: i32,
+
+    /// coordinates in the atlas of the top-left corner. These have to
+    /// be normalized to be between 0 and 1 prior to use in shaders.
+    pub atlas_x: u32,
+    pub atlas_y: u32,
+}