Add durable child-orchestration primitives with composable wait semantics

[Copilot]

pg_durable had no first-class way to express “start a child orchestration and durably resume when it completes.” This change adds graph-composable child-wait primitives so parent workflows can model sub-orchestration flows without backend-session polling or hand-rolled loop/sleep polling.

• New DSL primitives

  • Added df.await_instance(instance_id, timeout_seconds) as a durable future node that waits for another instance to reach a terminal state.
  • Added df.call_child(fut, label, options) as a convenience wrapper that starts a child via df.start(...) and then waits for it durably.
  • df.call_child currently supports:
    • timeout_seconds
    • database
    • on_failure = 'raise'

• Runtime / orchestration support

  • Introduced a new AWAIT_INSTANCE node type.
  • Added orchestration handling for AWAIT_INSTANCE using durable timer-based polling plus an activity that reads child instance state.
  • Success resolves to a JSON envelope containing:
    • instance_id
    • status
    • result
  • Failed / cancelled children and wait timeouts surface as parent errors.

• Compatibility / SQL surface

  • Updated df.ensure_durofut to recognize AWAIT_INSTANCE.
  • Updated df.grant_usage and the 0.2.0 -> 0.2.1 upgrade SQL so upgraded installs can grant and call the new functions immediately.

• Docs

  • Added a short design doc: docs/child-orchestration-design.md
  • Updated the user guide and upgrade notes for the new API and current semantics.

Example

SELECT df.start(
    df.call_child(
        'SELECT json_build_object(''report_id'', 42, ''status'', ''ready'')',
        'generate-report',
        '{"timeout_seconds": 300}'
    ) |=> 'child'
    ~> 'INSERT INTO audit_log(payload) VALUES ($child::jsonb)',
    'parent-workflow'
);

This expands to a child df.start(...) plus a durable wait, and returns the child envelope to the parent graph.