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/shape.rs

@@ -3,10 +3,9 @@
 //! The shaper implementation for the Linux backend is HarfBuzz
 //! ([`crate::shaper::harfbuzz::Shaper`]).
 
-use crate::collection::Index;
-
 pub use crate::shaper::cache::Cache;
 pub use crate::shaper::feature::{DEFAULT_FEATURES, Feature, FeatureList};
+pub use crate::shaper::run::{RunIterator, RunIteratorHook, RunOptions, TextRun};
 pub use crate::shaper::{harfbuzz, noop};
 
 /// Shaper implementation for our compile options (backend
@@ -46,26 +45,3 @@ pub struct Options {
     /// support applying features globally.
     pub features: Vec<String>,
 }
-
-/// A single text run. Port of `shaper/run.zig` `TextRun`.
-///
-/// TODO(integration): `run.zig` (RunIterator over terminal rows) is deferred
-/// to F2; the `grid: *SharedGrid` field is omitted here and should be added
-/// when `SharedGrid` and the run iterator are wired in.
-#[derive(Debug, Clone, Copy, PartialEq, Eq)]
-pub struct TextRun {
-    /// A unique hash for this run. This can be used to cache the shaping
-    /// results.
-    pub hash: u64,
-
-    /// The offset in the row where this run started. This is added to the
-    /// X position of the final shaped cells to get the absolute position
-    /// in the row where they belong.
-    pub offset: u16,
-
-    /// The total number of cells produced by this run.
-    pub cells: u16,
-
-    /// The font index to use for the glyphs of this run.
-    pub font_index: Index,
-}