fix(types): reserve type alias namespace names

[slepp]

Summary

• reserve top-level type alias names in the shared type namespace during collect_types
• fail closed on alias collisions before inserting into type_aliases
• add negative coverage for duplicate alias, alias-vs-struct, and alias-vs-trait collisions

Testing

• cargo test -p hew-types --quiet
• cargo clippy -p hew-types --tests -- -D warnings