Give the parent a fresh turn after a committed child start (child handle capability) #966

Need

A workflow should be able to start a child, get the committed child identity (child call, child instance, initial run), do other work, and later await the child's result. It should need no unrelated activity, timer, signal, selection winner or visibility query to get another parent turn after the child starts. This is the runtime dependency of sdk-php#185. The SDK contract is proposed in sdk-php#212, design doc. Other SDKs will want the same capability, so treat it as a portable contract, not a PHP-only convention.

Current native behavior (evidence)

Run on 2026-11-02 21:56:53–21:57:02 UTC. The unchanged probe scripts/probe-child-start-contract.php (sdk-php#212) used normally installed PHP SDK 3.6.2 (1c84f255) against ghcr.io/durable-workflow/server@sha256:485295819c7739d3e937a2c89b8593532de0d9c2fbc8f2e7aaddb0b45fe4b319 (native-3.0.1, source 923d45b3), with SQLite, role tokens and disposable containers:

• The parent's task completion carrying start_child_workflow commits ChildWorkflowScheduled and ChildRunStarted in the same transaction. Both carry child_call_id, the child instance ID and the initial run ID.
• The completion acknowledgment returns created_task_ids containing only the child's task, and no child identity.
• All 3 parent polls before child completion were empty. The parent gets a task only after the child resolves.
• In source, runtime/children.rs::start_child creates a workflow task for the child only. Cluster info advertises child_workflows, child_workflow_runtime_contract, child_workflow_retry_policy and child_workflow_timeouts, and none of them covers this.
• What's already correct and must be kept: child_call_id is stable through child continue-as-new while workflow_link_id changes; the typed result is preserved; a duplicate completion of the closed claim is refused (recorded=false, reason=already_completed).

This matches the published PHP Server 2.11 baseline recorded in sdk-php#185.

Proposed capability (names are proposals for the Server owner to settle)

1. Opt-in command mode. start_child_workflow accepts an explicit flag (for example "await": "started") that asks for a parent turn once the start is committed. Without the flag, the current result-waiting behavior and its task cost stay exactly as they are.
2. Fresh parent turn. In the same transaction that commits the child start, the runtime creates a new parent workflow task with a fresh claim (new task ID and lease). The poll that delivers it includes the committed ChildWorkflowScheduled/ChildRunStarted history. The child may remain open. The original claim stays closed: no further commands on it.
3. Duplicate safety. Duplicate acknowledgments, redelivery or a retried completion of the original task must not start a second child or create a second parent turn. That is the existing immutable task-receipt behavior, extended to the new task.
4. Identity is from canonical history only. The initial run ID is frozen from the original scheduling and start. Later continue-as-new or retries do not change what the turn reported. child_call_id is the stable call identity, not workflow_link_id.
5. Advertise and refuse. Advertise it in cluster info and in the worker-poll/complete server_capabilities, for example child_workflow_start_turn. A runtime without it must refuse the flagged command explicitly (a typed rejection before any child is started), never silently ignore an unknown option.
6. Policies unchanged. Parent-close policy (including Abandon), cancellation propagation, deadlines and stale-attempt fences behave exactly as for today's child start. Namespace: the child stays in the parent's namespace, and identity never crosses namespaces.

Out of scope here: caller-selected child IDs (workflow#850) and still-open children at parent continue-as-new (workflow#868).

Acceptance

• The flagged start produces exactly one child plus one fresh parent task while the child is still running; the turn's history carries the committed identity.
• Duplicate completion or redelivery of the original task produces no second child and no second turn.
• The unflagged start is unchanged (no extra parent task), shown with a before/after task count.
• The capability is advertised; a runtime without it refuses the flagged command before any side effect.
• Child continue-as-new and retries keep child_call_id and the initial run ID in the turn's history; the result resolves through the chain.
• After release, sdk-php re-runs its probe as a normally installed SDK against the published image.

Linked: sdk-php#185 (feature), sdk-php#212 (design and probe). Both are parked until this lands.