feat: add installable semantic test runner

[superposition]

Summary

• publish versioned manifest/frame contracts and validated Coatcheck.toml configuration
• add the installable coatcheck CLI with init, doctor, dev, and test
• generate Playwright + Axe semantic checks and review-only screenshots without pixel baselines
• package locked runtime templates and add README navigation for every maintained folder

Validation

• cargo fmt --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --all-targets --all-features
• cargo package --locked --allow-dirty
• Cold Front integration: 49 Playwright checks passed across 22 stories and 2 scenes

Release

Prepared as Coatcheck 0.2.0 for cargo install coatcheck --locked.