Improve TUI experience

[sakost]

Summary

The current terminal UI provides functional progress reporting but could be enhanced for a better user experience during long-running mutation testing sessions.

Current State

The TUI is event-driven with a background tokio render task (src/progress/):

• Render modes: Fancy (TTY default with progress bar + spinner), Plain (no color/overwrite), Verbose (per-mutant lines), Quiet (suppress all)
• Progress bar: Shows [#>-] pos/len mutants (ETA remaining) using indicatif
• Phase display: Animated spinner (⠋⠙⠹⠸⠼⠴⠦⠧⠇⠏) during coverage/generation phases, checkmark on completion
• Per-mutant output (verbose mode): Colored status + file + line + mutator + original → mutated text
• Final summary: Scoreboard with killed/survived/timeouts/errors/no_coverage counts

Suggested Improvements

Real-time statistics panel

• Show live operator-level kill rates as mutations run
• Show per-file progress (e.g., "file 3/12: src/auth/login.py")
• Display running mutation score that updates as results arrive

Interactive mode

• Pause/resume capability (Ctrl+Z or p key)
• Cancel with summary (currently Ctrl+C stops but could show partial results more prominently)
• Allow filtering/focusing on specific files mid-run

Better ETA estimation

• Use per-mutant timing history for more accurate ETA
• Show throughput (mutants/second)

Multi-line layout

• Dedicated sections for: current phase, progress bar, recent results, live stats
• Use ratatui or similar for a richer multi-panel layout

Log file support

• Option to write detailed per-mutant results to a log file while showing summary on screen
• Useful for CI where you want both a clean output and full details

Implementation

Key files:

• src/progress/reporter.rs — ProgressReporter (event sender)
• src/progress/event.rs — event types
• src/progress/render.rs — render loop
• src/progress/style.rs — colors and formatting

Currently uses console + indicatif crates.