Add macOS and Linux CI validation

[isty2e]

Summary

• add first-class CI for the primary Linux and macOS platforms
• run formatting and warning-denied Clippy on Ubuntu
• run the full locked test suite on Ubuntu and macOS
• run native release builds after pushes to main and manual dispatches
• expose the main workflow status in the README
• cancel superseded runs and keep workflow permissions read-only

Workflow ownership

• pull requests run quality checks and full tests
• pushes to main additionally run native release builds after each platform test
• the focused Windows no-replace workflow remains independent
• ARM builds and release asset publication remain in release-binaries.yml

Validation

• go run github.com/rhysd/actionlint/cmd/actionlint@latest .github/workflows/ci.yml
• cargo fmt --check
• cargo clippy --locked --all-targets -- -D warnings
• cargo test --locked (2,099 tests)
• cargo build --locked --release
• GitHub-hosted full test runs on Ubuntu and macOS