feat(packages): add Python and Rust releases

[connorcarpenter15]

Summary

• add publishable openengine-proto packages for Python and Rust with checked-in generated bindings
• add pinned, reproducible Python and Rust generation plus generated-code drift checks
• add package, MSRV, descriptor-set, and bidirectional Python/Rust wire compatibility tests
• add tag-driven PyPI, crates.io, and GitHub release automation
• document installation, compatibility, contribution, changelog, and release procedures

Why

Clients should be able to install OpenEngine bindings without cloning the schema repository or running Buf/protoc. A single release tag now coordinates both language packages and the canonical schema artifacts.

User and developer impact

• Python clients can use pip install openengine-proto
• Rust clients and engine implementations can use cargo add openengine-proto
• consumer builds do not invoke protoc
• schema changes must regenerate both checked-in packages
• releases require the external PyPI/crates.io ownership, Trusted Publisher, GitHub environment, and crates.io token setup documented in RELEASING.md

Validation

• Buf 1.71.0 lint and build
• generated binding drift check from a clean commit
• Python 3.10 wheel/sdist build, Twine metadata check, clean wheel install, and unit tests
• Rust tests on stable and the declared Rust 1.88 MSRV
• cargo package staged-archive rebuild
• bidirectional Python-to-Rust and Rust-to-Python serialization fixture
• Cargo fmt and Clippy with warnings denied
• actionlint 1.7.12
• Markdown lint and Lychee link checking