Fix df.loop continue_as_new restarting from graph root when loop is not root

[Copilot]

df.loop called continue_as_new inline in the main orchestration, so every new generation restarted from graph.root_node_id — re-executing prefix nodes on every loop iteration. For prefix ~> df.loop(body), the prefix ran once per iteration instead of once per instance.

A sub-orchestration approach was evaluated but is blocked by a duroxide 0.1.29 limitation: ContinueAsNew drops the parent link, so the parent orchestration never receives the child's completion.

Approach: loop-continue sentinel

Instead of calling continue_as_new inside execute_loop_node, the loop returns an upward-propagating sentinel that the root execute() handles:

// execute_loop_node — signals another iteration is needed
Ok(make_loop_continue_signal(node_id, results))

// Root execute() — calls continue_as_new from the correct scope
Ok(result) if is_loop_continue_signal(result) => {
    let (restart_node_id, cont_results) = extract_loop_continue(result);
    ctx.continue_as_new(FunctionInput {
        loop_continuation: Some(LoopContinuation { restart_node_id, results: cont_results }),
        ..
    }).await
}

On the next generation, execute() resumes from restart_node_id with pre-populated results, skipping any prefix nodes.

Changes

• src/orchestrations/execute_function_graph.rs

  • execute_loop_node: returns __loop_continue__ sentinel instead of calling continue_as_new
  • execute_then_node: propagates sentinel from RIGHT child unchanged; when sentinel comes from LEFT child, promotes restart_node_id to the THEN node itself so the right-side suffix is included in the restart path
  • Root execute(): detects sentinel, calls continue_as_new with LoopContinuation pointing at the loop node (or nearest enclosing THEN for loop ~> suffix)
  • Results serialized via BTreeMap for deterministic JSON key order — required for duroxide replay correctness
  • Removes the dead execute_loop / LOOP_NAME sub-orchestration

• src/types.rs: adds LoopContinuation struct and loop_continuation: Option<LoopContinuation> to FunctionInput (backward-compatible; absent on initial invocation)

• tests/e2e/sql/24_nonroot_loop.sql: regression tests for three non-root loop shapes:

  1. prefix ~> loop — prefix runs once, body runs N times
  2. prefix ~> loop ~> suffix — prefix and suffix each run once
  3. Named result from prefix node accessible inside loop body across generations