ci: expand platform coverage, enforce clippy warnings, add MSRV check
- Upgrade actions/checkout from v2/v4 to v6
- Add test-platforms job: ubuntu, ubuntu-arm, macos-arm, macos-intel, windows, windows-arm
- Add MSRV job using cargo-hack check --rust-version
- Replace baptiste0928/cargo-install with taiki-e/install-action@cross for cross-compilation
- Remove unnecessary Docker start step (cross handles container lifecycle)
- Enable clippy -D warnings with --all-targets (was disabled with TODO)
Lilith River
488da716d7