feat(test): add affected-crate fast Rust runner

[TheHypnoo]

Closes #7053

Summary

• add an opt-in local runner for Rust unit tests affected by tracked changes
• reuse CI's existing crate selection and target-mode logic
• preserve per-crate Cargo isolation and single-threaded runtime tests

Changes

File	Change
scripts/test_affected_crates.sh	Select affected crates from a base revision and run their fast unit-test targets
tests/test_affected_crates.sh	Verify paths, target modes, ordering, environment constraints, dry-run behavior, and invalid bases with fake tools
CONTRIBUTING.md	Document usage and replace the known-broken workspace test command

Test plan

• bash -n scripts/test_affected_crates.sh tests/test_affected_crates.sh
• shellcheck scripts/test_affected_crates.sh tests/test_affected_crates.sh
• ./tests/test_affected_crates.sh
• CARGO_BUILD_JOBS=1 cargo test --lib --bins -p perry-ui-testkit (5 passed)
• representative real --dry-run against origin/main
• git diff --check origin/main...HEAD

Limitations

• runs unit-test targets only; integration suites remain covered by scoped/nightly CI
• ignores untracked files until they are staged
• requires the selected base revision to exist locally

Checklist

• Linked approved issue
• Scripts pass ShellCheck
• Documentation updated
• Conventional commit format
• No attribution trailers
• Workspace version, CLAUDE.md, and CHANGELOG.md are unchanged

Summary by CodeRabbit

• New Features

  • Added a tool to run targeted Rust tests for crates affected by changes.
  • Added dry-run support to preview test commands before execution.
  • Added configurable base-revision support for determining affected files.

• Documentation

  • Updated source-building guidance with targeted testing instructions and CI scope details.

• Tests

  • Added coverage for affected-crate detection, command generation, dry runs, and invalid revisions.

[coderabbitai]

Note

Currently processing new changes in this PR. This may take a few minutes, please wait...

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 53978689-800d-4fa6-b389-e6d7fe29e403

📥 Commits

Reviewing files that changed from the base of the PR and between 788a757 and 5a643d3.

📒 Files selected for processing (3)

• CONTRIBUTING.md
• scripts/test_affected_crates.sh
• tests/test_affected_crates.sh

 ____________________________________
< LLM (Looking for Leaks in Memory). >
 ------------------------------------
  \
   \   (\__/)
       (•ㅅ•)
       / 　 づ

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/affected-crate-fast-tests

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands.}