Work admission and shutdown completion

lifecycle.WorkLifetime owns admitted work and one shutdown result. Request contexts control request work; a shutdown caller's context controls only how long that caller waits. Canceling a waiter does not dispose resources underneath accepted work.

BeginStop(dispose) seals admission, signals the owned work to stop, waits for its tickets to return, and runs the first disposer exactly once. Use WaitStopped(ctx) for a bounded wait or JoinStopped() to wait for actual retirement without a caller deadline. Both report the same retained outcomes. JoinStopped does not initiate shutdown; it can also join a stop another owner has already initiated.

An owning component's unconditional Close uses the existing completion, without inventing a cancellation context:

func (c *component) Close() error {
    c.work.BeginStop(c.closeNative)
    return c.work.JoinStopped()
}

A supervisor can call BeginStop on every sibling before joining any of them. For a bounded caller, Shutdown(ctx, dispose) still starts the same protocol and waits using the caller's budget. A later caller can join its actual completion after that budget expires.

Context() is the owned work context, canceled when stopping begins. Passing it to WaitStopped can end the wait before retirement; use JoinStopped when actual completion is required. An active worker must return its ticket before joining its own lifetime.

OnceCompletion uses the same distinction: Do executes the winning operation in its caller's frame and joins completion; Go dispatches it; DoContext dispatches it and bounds only the caller's wait. The operation retains the context its own owner supplied. Repeated calls do not repeat the operation.