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/atlas.rs

@@ -14,8 +14,10 @@
 //!     that regions written INTO the atlas do not have to be square, only
 //!     the full atlas texture itself.
 
-use std::io::Write;
-use std::sync::atomic::{AtomicUsize, Ordering};
+use std::{
+    io::Write,
+    sync::atomic::{AtomicUsize, Ordering},
+};
 
 /// The format of the texture data being written into the Atlas. This must be
 /// uniform for all textures in the Atlas. If you have some textures with