Release Rust SDK 2.0.3

[rmcdaniel]

Publish the runtime external payload resolution fix from #37. Package version, product metadata, changelog and release-contract test advance together; no new dependency or protocol revision.

The source fix passed Rust 1.86/stable, official Avro cross-language checks, regression corpus, fresh-package consumer and docs checks. A candidate worker recovered the original stalled Server 2.3.0 workflow without another activity execution or failed workflow task. #36 remains open until immutable publication and the published-SDK storage-pressure/restart rerun succeed.

The release will use the repository publication workflow and verify the registry archive against this source commit.

[rmcdaniel]

Focused release-entrypoint tests pass: 10 tests. The local invocation must put the pinned release-tooling venv on PATH because its mocked shell subprocesses invoke python3; invoking only the venv interpreter was insufficient and produced a missing markdown_it error. Corrected without product changes. Public release-candidate Rust matrices, corpus, package and docs checks are green; official Avro qualification is finishing.

[rmcdaniel]

Published Rust SDK 2.0.3 from b77dcfebf0c0c406b9ac02b029fca6f596038af4 at 01:55 UTC. Release and publication verification are complete: the registry archive/source identity and fresh consumer build passed. Server #136 now pins the released crate for its fresh three-SDK recovery/restart experiment.