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-config/src/metric_modifier.rs

@@ -81,7 +81,8 @@ impl MetricModifier {
     #[must_use]
     pub fn apply_i32(&self, v: i32) -> i32 {
         let r = self.apply_i64(i64::from(v), false);
-        i32::try_from(r).unwrap_or(if r < 0 { i32::MIN } else { i32::MAX })
+        // Mirrors Zig: out-of-range yields maxInt * sign.
+        i32::try_from(r).unwrap_or(if r < 0 { -i32::MAX } else { i32::MAX })
     }
 
     /// Apply to a float value.