Document local Rust checks

[kriszyp]

Update CLAUDE.md Testing to require cargo test, cargo clippy --all-targets, and cargo fmt --check before pushing Rust changes, and point Conventions at the all-target Clippy command. Testing also records the pinned toolchain and PATH requirements, the addon rebuild before Node tests, and the Linux checks required before macOS pushes.

For the human reviewer

1. Requirement and layer. This guidance is warranted because npm test does not run the Rust checks CI enforces. I kept the checks in CLAUDE.md instead of adding a package script or pre-push hook, which would exceed the requested documentation-only scope. That choice is easy to revise, but it relies on contributors following the instructions; CI still catches misses.
2. Platform check. macOS test and Clippy runs do not compile the Linux-only branches CI checks, so the instructions require running both commands in Linux before pushing from macOS. The existing Docker images are linked from the README, and cargo fmt --check works on either platform. This adds a Linux environment step but catches platform-specific failures before PR CI.
3. Rustup fallback. I kept rustup toolchain install without a toolchain argument: rustup 1.29.1 accepts that form and installs the active toolchain, which this repo defines in rust-toolchain.toml. The Rustup 1.28.0 release notes document the no-argument behavior; rustup show reports the active toolchain but does not install it.

Verification

• From the repository root, cargo --version reported 1.94.1.
• cargo test: 144 passed, 0 failed.
• cargo clippy --all-targets: passed.
• cargo fmt --check: passed.
• git diff --check: passed.
• npm run build:debug and npm test were not run because this change is documentation-only; no source or generated addon code changed.

Complexity: easy

🤖 Generated with OpenAI Codex

_{Review-Coverage: authored=codex; ran=gemini,claude; adjudicated=domain; declined=cursor-grok,cursor-kimi,cursor-muse,cursor-composer; rounds=6; full=1 @ bd48172}

_{Human-Review-Need: 3 (decisions: local-linux-before-push, contributor-guidance-layer) @ bd48172}

[gemini-code-assist]

Code Review

This pull request updates CLAUDE.md with detailed instructions on Rust toolchain setup, local testing, and linting guidelines. The review feedback points out two issues in the documentation: a command error where running rustup toolchain install without arguments fails (suggesting rustup show instead), and a factual inaccuracy regarding how macOS handles Linux-specific compilation branches.