Add graceful shutdown and cancellation for actors and background tasks

[artrixdotdev]

Problem

A full TUI needs deterministic shutdown. Some background tasks and actor children are killed directly or spawned without a public cancellation/shutdown surface.

Relevant code:

• Torrent actor stop kills peers/trackers/store/scheduler: crates/libtortillas/src/torrent/actor.rs:572
• UDP server starts an unmanaged receiver task: crates/libtortillas/src/tracker/udp.rs:403
• Swarm peer connections use tokio::spawn: crates/libtortillas/src/torrent/swarm.rs:30

Dependencies

Blocked by #225 for public lifecycle semantics.
Blocked by #232 because engine startup/shutdown should both be explicit frontend-visible phases.

Acceptance criteria

• Engine shutdown stops torrents, trackers, peers, schedulers, UDP receiver tasks, and outstanding connection tasks.
• Shutdown is awaitable and returns typed errors for failures/timeouts.
• Repeated shutdown calls are safe.
• No task leaks are expected after shutdown.
• Tests cover graceful shutdown of an engine with at least one torrent actor.