feat(rust-client): add official Rust SDK

[starboyate]

Summary

• Add the official async Rust SDK for Memind Open API, including typed request/response models, structured errors, retries, and safe URL handling.
• Add Rust client CI and a manual crates.io release workflow.
• Document the Rust client in the root README and exclude Rust files from Maven license/checkstyle scanning.

Test Plan

• cargo fmt --check
• cargo +1.86.0 check --lib --all-features
• cargo check --all-targets --all-features
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --all-features
• cargo package
• mvn com.mycila:license-maven-plugin:check

Notes

• The branch is based on the latest origin/main and does not include docs/superpowers changes.
• mvn checkstyle:check currently fails on pre-existing Java client checkstyle violations unrelated to this Rust SDK change.