End-to-end validation and migration

[KedoKudo]

Task

Verify full workflow produces identical output to Python CLI. Set up dual-launch (pixi run start = Python Qt GUI, pixi run gui = Rust egui GUI). Document migration path.

Coexistence

• pixi run start = Python Qt GUI (unchanged)
• pixi run gui = Rust egui GUI
• pixi run cli = Python CLI (gains --rust-backend flag)

Feature parity checklist

• All Python unit tests pass with Rust backend
• demo_config workflow produces identical CSV
• Session files are JSON-compatible
• Performance equal or better

Part of Epic #354