Make Engine startup async and fallible

[artrixdotdev]

Problem

Engine::new is synchronous and returns Self, but actor startup binds TCP/uTP/UDP sockets asynchronously. Startup failures are not clearly surfaced to the frontend.

Relevant code:

• Public constructor: crates/libtortillas/src/engine/mod.rs:95
• Socket binds in actor startup: crates/libtortillas/src/engine/actor.rs:137

Dependency

Blocked by #231 because startup failures need typed public errors.

Acceptance criteria

• Engine construction/startup is async and returns Result.
• TCP/uTP/UDP bind failures are reported before the frontend treats the engine as usable.
• Default construction semantics are documented.
• Existing examples and doctests are updated.
• Tests cover bind failure behavior where feasible.