Spec v0.9.0 compliance

Tracks [spec v0.9.0](https://github.com/open-feature/spec/releases/tag/v0.9.0) compliance. Functionally complete; only harness bump remains.

- [ ] Bump `spec` submodule to the v0.9.0 tag; re-run gherkin e2e.

- [ ] Provider lifecycle event ownership (race-condition fix): add opt-in marker so providers emit their own `PROVIDER_READY`/`PROVIDER_ERROR`; SDK derives status from the provider event stream and keeps + deprecates the legacy synthetic path; update first-party providers ([spec 5.3.5](https://openfeature.dev/specification/sections/events#requirement-535), [provider status 2.8](https://openfeature.dev/specification/sections/providers#28-provider-status), [Appendix E](https://openfeature.dev/specification/appendix-e))

Start with the spec v0.9.0 release, the spec submodule, and the gherkin end-to-end harness. Review the linked provider lifecycle requirements and first-party providers, then implement the opt-in provider event ownership and legacy-path deprecation; done means the submodule is at v0.9.0 and the gherkin e2e checks pass.

Written by the indexing model from the issue text.