Execute dispatched workflows across distributed nodes

## Parent

#674

## What to build

Prove that DispatchWorkflow remains transport-neutral in a clustered host. Use the existing distributed Groundwork execution actor provider, durable command transport, placement leases, and fencing so that a child-start intent produced on one node may be claimed and executed by an eligible node without changing the activity contract or introducing broker-specific concepts.

Covers parent user stories 45, 47–49, and 53–54.

## Acceptance criteria

- [ ] A two-node acceptance scenario proves that a dispatch checkpoint committed on one node can result in child execution on an eligible node.
- [ ] The existing workflow start dispatcher and configured execution actor provider remain the only activity-facing execution seam.
- [ ] Placement and forwarding choose the draining node, while checkpoint fencing remains the safety boundary against stale or duplicate writers.
- [ ] Duplicate child-start delivery and placement changes converge on one logical child execution.
- [ ] Restarting either node after durable intent creation preserves dispatch and child-start progress.
- [ ] Dispatch lifecycle state and authenticated inspection remain consistent regardless of the node executing the child.
- [ ] Deployment readiness distinguishes in-memory development, durable single-node Groundwork, and distributed Groundwork compositions.
- [ ] Architecture tests prove no MassTransit, service-bus, routing-channel, priority, affinity, or transport-selection dependency enters the activity contract.
- [ ] Existing local in-process dispatch behavior remains unchanged.

## Blocked by

- #676
- #678

Start with the existing distributed Groundwork execution actor provider, workflow start dispatcher, durable command transport, placement leases, and fencing. Run or inspect the existing local in-process dispatch behavior and architecture tests before designing the two-node acceptance scenario. Done means durable dispatch survives restart, placement changes and duplicate delivery converge on one child execution, while the activity contract stays transport-neutral.

Written by the indexing model from the issue text.