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-bench/src/main.rs

@@ -1,7 +1,8 @@
 //! `ghostty-bench +<action> [--option=value ...]` (port of `main_bench.zig`).
-use ghostty_bench::{args::Args, benches};
 use std::process::ExitCode;
 
+use ghostty_bench::{args::Args, benches};
+
 fn main() -> ExitCode {
     let argv: Vec<String> = std::env::args().skip(1).collect();
     let mut action = None;