Add a TUI-style integration example for commands, events, and snapshots

[artrixdotdev]

Problem

Before building the actual TUI, libtortillas should include a small example showing the intended frontend consumption pattern. This prevents the TUI from inventing an accidental architecture around polling internals.

Dependencies

Blocked by #225, #227, and #228 because the example should use the lifecycle, event, and snapshot APIs.

Example should demonstrate

• Creating/starting the engine
• Adding a torrent from typed input
• Sending lifecycle commands
• Subscribing to events
• Reading periodic snapshots
• Shutting down cleanly

Acceptance criteria

• Add a crate example or documented no-run example showing a TUI-like loop.
• Example does not import actor/protocol internals.
• Example compiles in doctests or CI where practical.
• The actual TUI can use the example as its initial architecture reference.