Add CI job for Linux g++ with latest nightly Rust

[Copilot]

• Identify CI failure: E0446 private trait ZngParser in public interface
• Root cause: pub trait MatchableParse and pub trait MatchPatternParse in private conditional module use pub(crate) ZngParser in RPITIT return types
• Fix: change pub trait MatchableParse and pub trait MatchPatternParse to pub(crate) (the conditional module is private, so pub was incorrect)
• Verify: all root crates pass cargo +nightly check
• Verify: cargo fmt -- --check passes