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/Cargo.toml

@@ -15,6 +15,8 @@ ghostty-unicode.workspace = true
 bitflags = "2.13.2"
 yeslogic-fontconfig-sys = "6.0.1"
 ghostty-datastruct.workspace = true
+ghostty-config.workspace = true
+ghostty-terminal.workspace = true
 
 # This crate wraps the freetype / harfbuzz / fontconfig C libraries through
 # their `-sys` crates; unsafe is denied by default and allowed on minimal,
@@ -47,3 +49,6 @@ return_self_not_must_use = "allow"
 used_underscore_binding = "allow"
 large_stack_arrays = "allow"
 inline_always = "allow"
+
+[dev-dependencies]
+ghostty-vt.workspace = true