feat(font): add run iterator, shared grid set and config integration

Completes the font engine port:
- shaper/run.rs ports run.zig: the RunIterator over terminal rows that
  splits text runs at style/face boundaries, selection and cursor,
  handles graphemes and ligatures; Shaper::shape_run shapes a run.
- shared_grid_set.rs ports SharedGridSet.zig: grids keyed by a config
  derived key, handed out as Arc<SharedGrid>.
- Font-related types now have one canonical definition in ghostty-config
  (Variation, Descriptor, CodepointMap, metric modifiers, freetype load
  flags, synthetic style, shaping break) which the font crate re-exports.
  MetricModifier::apply_i32 now saturates to -i32::MAX like Zig.
- The 23 terminal-driven harfbuzz shaper tests are ported, printing via
  Screen until Terminal lands.

Test Plan:
- cargo test -p ghostty-font (157 passed, 3 ignored - also skipped in
  Zig); Devanagari/Chakma/Bengali tests return early without system fonts
- cargo test -p ghostty-config (255 passed)

crates/ghostty-font/src/lib.rs

@@ -24,6 +24,7 @@ pub mod opentype;
 pub mod shape;
 pub mod shaper;
 pub mod shared_grid;
+pub mod shared_grid_set;
 pub mod sprite;
 
 pub use atlas::Atlas;
@@ -40,6 +41,7 @@ pub use library::Library;
 pub use metrics::Metrics;
 
 pub use shared_grid::SharedGrid;
+pub use shared_grid_set::SharedGridSet;
 pub use sprite::{Face as SpriteFace, Sprite};
 
 /// The styles that a family can take.