Adapter boundary for executing one runtime graph node activation.

Executors run node code; they must not mutate the run, apply writes, emit checkpoints, decide graph termination, or read uncommitted superstep writes. The dispatcher normalizes raises, exits, and throws, so executors may let node exceptions propagate.

v0.1 ships Docket.Executor.Local, which executes inside the dispatcher's isolated, finite-deadline activation process. Custom executors receive the same hard outer boundary. Queue, remote, and late-completion protocols are post-v0.1; the {:await, term()} return is reserved for them and is treated as a permanent node failure in v0.1.

The runtime dispatches all activations in a superstep concurrently. The executor callback remains a single-activation boundary; the update barrier waits for every callback and applies their results in deterministic activation order.