style: apply nightly rustfmt import grouping across the workspace

rustfmt.toml sets group_imports, imports_granularity and imports_layout,
which are nightly-only; agents formatted with stable rustfmt, which
ignored them. This is a formatting-only pass with `cargo +nightly fmt`.

Test Plan:
- cargo clippy --workspace --all-targets -- -D warnings
- cargo test --workspace

crates/ghostty-font/src/metrics.rs

@@ -1,7 +1,6 @@
 //! Font metrics for the terminal grid. Port of `font/Metrics.zig`.
 
-use std::collections::HashMap;
-use std::str::FromStr;
+use std::{collections::HashMap, str::FromStr};
 
 /// Metrics describing the terminal grid cell for a font.
 #[derive(Debug, Clone, Copy, PartialEq)]