Computation and concurrency

This trunk starts where more than one thing can make progress, where ordering stops being obvious, and where resource lifetime becomes part of correctness.

Start with Concurrency for the model and Cancellation for the moment a running computation has to give ownership back. Virtual machines as authority over real resources follows the same ownership question down into CPU, memory, IOMMU, and physical-device assignment: what remains virtual when the guest is using real hardware directly?

This region connects quickly to retries and timeouts, idempotency, runtime lifetimes, and authority.