fix(set-version): reject unmatched package selectors

[ychampion]

Summary

• reject explicit package selectors that do not match a workspace member
• validate the complete selection before changing any manifests
• cover single, mixed, multiple, and repeated missing selectors

Why

A nonexistent --package currently exits successfully without doing anything. Mixed valid and invalid selectors can also update part of a workspace before the mistake is noticed.

Closes #888.

Validation

• cargo test --locked --all-features
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo fmt --all -- --check
• built cargo-set-version CLI smoke tests for missing and valid selectors

[epage]

Thanks!