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/draw/geometric_shapes.rs

@@ -0,0 +1,143 @@
+//! Geometric Shapes | U+25A0...U+25FF
+//! <https://en.wikipedia.org/wiki/Geometric_Shapes_(Unicode_block)>
+//!
+//! ■ □ ▢ ▣ ▤ ▥ ▦ ▧ ▨ ▩ ▪ ▫ ▬ ▭ ▮ ▯
+//! ▰ ▱ ▲ △ ▴ ▵ ▶ ▷ ▸ ▹ ► ▻ ▼ ▽ ▾ ▿
+//! ◀ ◁ ◂ ◃ ◄ ◅ ◆ ◇ ◈ ◉ ◊ ○ ◌ ◍ ◎ ●
+//! ◐ ◑ ◒ ◓ ◔ ◕ ◖ ◗ ◘ ◙ ◚ ◛ ◜ ◝ ◞ ◟
+//! ◠ ◡ ◢ ◣ ◤ ◥ ◦ ◧ ◨ ◩ ◪ ◫ ◬ ◭ ◮ ◯
+//! ◰ ◱ ◲ ◳ ◴ ◵ ◶ ◷ ◸ ◹ ◺ ◻ ◼ ◽︎◾︎◿
+//!
+//! Only a subset of this block is viable for sprite drawing; filling
+//! out this file to have full coverage of this block is not the goal.
+//!
+//! Port of `sprite/draw/geometric_shapes.zig`.
+
+use super::common::{Corner, DrawFn, Range, Shade, Thickness};
+use crate::metrics::Metrics;
+use crate::sprite::canvas::{Canvas, CapMode, Color, DrawError, Path, StrokeOptions};
+
+pub const RANGES: &[Range] = &[
+    Range {
+        min: 0x25e2,
+        max: 0x25e5,
+        draw: draw25e2_25e5 as DrawFn,
+    },
+    Range {
+        min: 0x25f8,
+        max: 0x25fa,
+        draw: draw25f8_25fa as DrawFn,
+    },
+    Range {
+        min: 0x25ff,
+        max: 0x25ff,
+        draw: draw25ff as DrawFn,
+    },
+];
+
+/// ◢ ◣ ◤ ◥
+pub fn draw25e2_25e5(
+    cp: u32,
+    canvas: &mut Canvas,
+    _width: u32,
+    _height: u32,
+    metrics: &Metrics,
+) -> Result<(), DrawError> {
+    match cp {
+        // ◢
+        0x25e2 => corner_triangle_shade(metrics, canvas, Corner::Br, Shade::ON),
+        // ◣
+        0x25e3 => corner_triangle_shade(metrics, canvas, Corner::Bl, Shade::ON),
+        // ◤
+        0x25e4 => corner_triangle_shade(metrics, canvas, Corner::Tl, Shade::ON),
+        // ◥
+        0x25e5 => corner_triangle_shade(metrics, canvas, Corner::Tr, Shade::ON),
+
+        _ => unreachable!(),
+    }
+}
+
+/// ◸ ◹ ◺
+pub fn draw25f8_25fa(
+    cp: u32,
+    canvas: &mut Canvas,
+    _width: u32,
+    _height: u32,
+    metrics: &Metrics,
+) -> Result<(), DrawError> {
+    match cp {
+        // ◸
+        0x25f8 => corner_triangle_outline(metrics, canvas, Corner::Tl),
+        // ◹
+        0x25f9 => corner_triangle_outline(metrics, canvas, Corner::Tr),
+        // ◺
+        0x25fa => corner_triangle_outline(metrics, canvas, Corner::Bl),
+
+        _ => unreachable!(),
+    }
+}
+
+/// ◿
+pub fn draw25ff(
+    _cp: u32,
+    canvas: &mut Canvas,
+    _width: u32,
+    _height: u32,
+    metrics: &Metrics,
+) -> Result<(), DrawError> {
+    corner_triangle_outline(metrics, canvas, Corner::Br)
+}
+
+fn corner_path(metrics: &Metrics, corner: Corner) -> Path {
+    let float_width = f64::from(metrics.cell_width);
+    let float_height = f64::from(metrics.cell_height);
+
+    let (x0, y0, x1, y1, x2, y2) = match corner {
+        Corner::Tl => (0.0, 0.0, 0.0, float_height, float_width, 0.0),
+        Corner::Tr => (0.0, 0.0, float_width, float_height, float_width, 0.0),
+        Corner::Bl => (0.0, 0.0, 0.0, float_height, float_width, float_height),
+        Corner::Br => (
+            0.0,
+            float_height,
+            float_width,
+            float_height,
+            float_width,
+            0.0,
+        ),
+    };
+
+    let mut path = Path::new();
+    path.move_to(x0, y0);
+    path.line_to(x1, y1);
+    path.line_to(x2, y2);
+    path.close();
+    path
+}
+
+pub fn corner_triangle_shade(
+    metrics: &Metrics,
+    canvas: &mut Canvas,
+    corner: Corner,
+    shade: Shade,
+) -> Result<(), DrawError> {
+    let path = corner_path(metrics, corner);
+    canvas.fill_path(&path, Color(shade.0))
+}
+
+pub fn corner_triangle_outline(
+    metrics: &Metrics,
+    canvas: &mut Canvas,
+    corner: Corner,
+) -> Result<(), DrawError> {
+    let float_thick = f64::from(Thickness::Light.height(metrics.box_thickness));
+    let path = corner_path(metrics, corner);
+
+    canvas.inner_stroke_path(
+        &path,
+        StrokeOptions {
+            line_cap_mode: CapMode::Butt,
+            line_width: float_thick,
+        },
+        Color::ON,
+    )
+}