hotfix: add cap to clap_lex version and install with --locked flag

[martinabeleda]

Fixes #27

cargo install ripdiff doesn't use the lockfile, so Cargo resolves clap_lex to 1.1.0 which requires Rust edition 2024. Users with Cargo older than Rust 1.85 can't parse this dependency.

Changes made:

1. Cargo.toml — Added clap_lex = ">= 1.0.0, < 1.1.0" as a direct dependency to cap the version and prevent resolution to the edition-2024 version.
2. README.md — Changed install command to cargo install --locked ripdiff so the published lockfile is respected, preventing unexpected dependency upgrades.

The --locked flag is the most robust fix since it ensures the exact tested dependency versions are used. The clap_lex cap is a belt-and-suspenders safeguard for users who forget --locked.