[huacnlee]

Summary

• add rust_i18n::extend!(crate_name) so applications can lazily merge a crate-scoped translation namespace into dependencies while dependencies keep using t!
• let application values override the same locale/key, while omitted values continue to come from the dependency and then normal locale fallback
• isolate each extension to the crate identifier passed to extend!, including Cargo aliases, without copying the application backend
• add runnable examples for renamed (my_component) and default (ui_component) dependency names

Test Plan

• cargo fmt --all -- --check
• cargo test --workspace
• cargo test --doc
• cargo test -p extend-crate-app -p extend-crate-default-app
• cargo run -p extend-crate-app
• cargo run -p extend-crate-default-app

Note: cargo clippy --workspace --all-targets -- -D warnings reaches two pre-existing warnings outside this diff in crates/support/src/backend.rs:166 and crates/support/src/lib.rs:132.