Add timeout and cancellation effects to the effect system.

Motivation

Agent workloads constantly need bounded execution: limiting API calls to N seconds, cancelling in-flight work when a parent task completes, racing multiple operations and returning the first result. These compose naturally as algebraic effects.

Proposed effects

fn fetch_with_timeout(@String -> Result<@String, @String>)
  effects(<Http, Timeout>)
{
  Timeout.with_deadline(5000, fn(-> Result<@String, @String>) effects(<Http>) {
    Http.get(@String.0)
  })
}

• Timeout.with_deadline(ms, thunk) — run thunk with a wall-clock deadline
• Timeout.cancel() — cancel the current operation
• Effect handlers at the host level enforce actual time limits via wasmtime fuel/epoch interruption

Dependencies

• Async futures and promises (<Async>) #59 (Async) — timeout semantics depend on the concurrency model