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/Cargo.toml

@@ -5,9 +5,10 @@ edition.workspace = true
 license.workspace = true
 
 [dependencies]
-ghostty-config.workspace = true
-ghostty-unicode.workspace = true
-ghostty-terminal.workspace = true
+tiny-skia = "0.12.0"
+png = "0.18.1"
+thiserror = "2.0.21"
+log = "0.4.34"
 
 [lints]
 workspace = true