v0.9.0 — Ecosystem Ready

feat: v0.9.0 Ecosystem Ready

Python 3.13-3.15 + free-threaded CPython support, allow_duplicate_keys, SerializeOptions expansion (width / indent_mapping / indent_sequence / indent_offset), tag handler registry with chaining + remove_tag, Pydantic integration via parse_as(), committed .pyi stubs, and a CI optimization (single rust-lint job, wheel install instead of maturin develop).

Review follow-up fixes: wired the previously dead serializer indent options, added a width=1 hang guard, i18n'd duplicate-key errors across 4 locales, and made non-string tag handler returns raise YamlTagError.