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-unicode/src/lut.rs

@@ -14,8 +14,7 @@
     clippy::expect_used
 )]
 
-use std::collections::HashMap;
-use std::fmt::Write as _;
+use std::{collections::HashMap, fmt::Write as _};
 
 const BLOCK_SIZE: usize = 256;