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/lib.rs

@@ -1 +1,14 @@
-//! ghostty-font: see docs/rewrite/ARCHITECTURE.md for the module map.
+//! Font engine. Currently contains the metrics, atlas, glyph and sprite-font
+//! modules; the rest of the font engine (faces, shaping, collections, ...) is
+//! ported later.
+
+#![cfg_attr(test, allow(clippy::unwrap_used))]
+
+pub mod atlas;
+pub mod glyph;
+pub mod metrics;
+pub mod sprite;
+
+pub use atlas::Atlas;
+pub use glyph::Glyph;
+pub use metrics::Metrics;