ci: strengthen Rust quality gates

[kholdrex]

Summary

Strengthens the Rust CI workflow so pull requests run the same deterministic quality gates used for local validation. The workflow now checks formatting, Clippy, all-feature tests, and example compilation without running the longer example programs.

Changes

• Replaces the manual rustup bootstrap with a Rust toolchain action that installs rustfmt and Clippy.
• Updates checkout to actions/checkout@v4 and adds Rust dependency caching.
• Adds CI steps for formatting, Clippy with warnings denied, all-feature tests, and all-feature example compilation.

Test Plan

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --all-features
• cargo check --examples --all-features
• git diff --check