docs: fix import order convention to match cargo fmt

cargo fmt sorts external crate and std imports alphabetically
together — it does not enforce a separate "external before std"
grouping. Update CONTRIBUTING.md and CLAUDE.md to reflect the
actual tooling behavior.

https://claude.ai/code/session_01MHKaZm6DcA9kjBPnW3KN5K

Author: claude

CLAUDE.md

@@ -6,7 +6,7 @@ Read and follow the CONTRIBUTING.md file in this repository for all code style c
 
 - Commit format: Conventional Commits — `type(scope): lowercase description`
 - Version releases are the only exception: just the version number (e.g. `0.21.1`)
-- Import order: internal (`crate::`/`super::`) → external crates → `std::`, prefer merged imports
+- Import order: internal (`crate::`/`super::`) first, then external crates and `std::` together (alphabetical, enforced by `cargo fmt`), prefer merged imports
 - Use descriptive generic names (`Size`, `Report`), not single letters
 - Prefer `where` clauses for multiple trait bounds
 - Derive order: std traits → comparison traits → `Hash` → derive_more → feature-gated

CONTRIBUTING.md

@@ -37,10 +37,9 @@ Prefer **merged imports** — combine multiple items from the same crate or modu
 Imports are grouped in this order, separated by blank lines:
 
 1. `use super::...` or `use crate::...` (internal)
-2. External crate imports (alphabetical)
-3. `use std::...` (standard library)
+2. All other imports — external crates and `std::` together, sorted alphabetically
 
-Within each group, items are ordered alphabetically. Platform-specific imports (`#[cfg(unix)]`) go in a separate block after the main imports.
+`cargo fmt` enforces alphabetical ordering across external and `std` imports (it does not distinguish between them). Platform-specific imports (`#[cfg(unix)]`) go in a separate block after the main imports.
 
 ```rust
 use crate::{