Run Rust formatting, tests, and Clippy in CI

Context

The CI jobs named “Format, check, and test examples” run ./scripts/test.py --operation validate:

• .github/workflows/ci.yml:27-44
• .github/workflows/ci.yml:46-62

That validation runs Roc tests/format/check operations, but it does not run Rust formatting, Rust tests, or Clippy. At commit 295000a during the release-candidate audit:

• cargo check --locked --all-targets passed;
• cargo fmt --all -- --check failed on formatting in src/lib.rs;
• cargo test --locked --all-targets passed but ran zero Rust tests;
• cargo clippy --locked --all-targets -- -D warnings failed on generated acronym names and a raw-pointer ABI lint.

The generated src/roc_platform_abi.rs needs targeted lint allowances rather than disabling useful linting for handwritten host code.

Suggested direction

Add a dedicated Rust validation job or extend the current validation job to run:

• cargo fmt --all -- --check
• cargo test --locked --all-targets
• cargo clippy --locked --all-targets -- -D warnings

Configure narrow allowances for generated glue. Add unit tests for pure host helpers and error mappings so cargo test provides meaningful coverage in addition to end-to-end Roc examples.

Acceptance criteria

• Rust formatting is required by CI and the current formatting difference is fixed.
• Handwritten Rust passes Clippy with warnings denied.
• Generated glue has documented, narrowly scoped lint allowances.
• Rust tests cover representative HTTP method/error mapping, TCP read helpers, SQLite lifecycle helpers, and filesystem classification.
• Contributor verification instructions list the same commands CI runs.

Start with the two validation jobs in .github/workflows/ci.yml:27-44 and 46-62, then inspect ./scripts/test.py --operation validate and src/lib.rs. Run the listed cargo fmt, cargo test, and cargo clippy commands to reproduce current failures; done means CI enforces them, generated src/roc_platform_abi.rs has narrow documented allowances, and the requested host-helper tests and contributor instructions are present.