Migrate completest-pty from vt100 to rio-vt

[raphamorim]

Swaps the terminal parser in completest-pty from vt100 to rio-vt, Rio's terminal engine. comptest now feeds the shell's PTY output through a rio-vt Crosswords + Processor and reads the screen back with format(FormatOptions::plain()) — same captured-completion string as before, no behavior change.

rio-vt is a safe-Rust, dependency-light terminal core (VT state machine, grid with scrollback, parser) and is actively maintained. It also drops the transitive net2 (RUSTSEC-2020-0016, unmaintained) that the old tree pulled in, so cargo deny check advisories stays clean.

• Benchmark against vt100 and alacritty_terminal: https://github.com/raphamorim/rio-vt-benchmark
• Background on the engine: https://rioterm.com/blog/2026/07/27/rio-vt-and-librio