feat(terminal): add libghostty core adapter

[cpendery (cpendery)]

Summary

• add the optional libghostty Rust feature and pinned libghostty-rs dependency
• translate Ghostty cells, colors, scrollback, cursor state, and PTY replies into the neutral terminal grid
• install Zig 0.16 for all-features CI builds

Validation

• cargo clippy -p tui-test-rs --all-targets --features libghostty -- -D warnings
• cargo test -p tui-test-rs --features libghostty

Signed-off-by: cpendery cpendery@vt.edu --- _{Stack created with GitHub Stacks CLI • Give Feedback 💬}