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-termio/src/backend.rs

@@ -6,10 +6,12 @@ use std::sync::Arc;
 use ghostty_renderer::size::{GridSize, ScreenSize};
 use ghostty_terminal::terminal::Terminal;
 
-use crate::exec::{self, Exec};
-use crate::sinks::SurfaceMailbox;
-use crate::state::SharedState;
-use crate::termio::Output;
+use crate::{
+    exec::{self, Exec},
+    sinks::SurfaceMailbox,
+    state::SharedState,
+    termio::Output,
+};
 
 /// The kinds of backends.
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]