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-datastruct/src/split_tree.rs

@@ -20,9 +20,10 @@
 //! parameters / `Allocator.Error`), the arena is replaced by a `Vec`, and the
 //! GObject boxed-type glue (`getGObjectType`) is left to the GTK crate.
 
-use half::f16;
 use std::fmt;
 
+use half::f16;
+
 /// The requirements on the view type stored in a [`SplitTree`].
 pub trait SplitTreeView: Clone {
     /// Return a label that is used for the debug view. If this returns