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Description

## Problem

The ForEachOperator in engine/interpreters/operators.py:614 uses a sequential for loop:

for idx, item in enumerate(items):
    iteration_result = self._execute_loop_body_task(ctx, body_task)

This means all iterations run one-by-one, not in parallel. Evidence from arxiv_summarization workflow:
- Extract PDF 1: 08:48:24
- Extract PDF 2: 08:48:46 (+22 sec)
- Extract PDF 3: 08:49:11 (+25 sec)

If parallel, all would start within milliseconds.

## Expected Behavior

ForEach should support parallel execution similar to ParallelOperator, which spawns separate workflow branches.

## Options

1. Add parallel mode to ForEach: builder.foreach(..., parallel=True) spawns concurrent branches
2. Document current behavior: ForEach is sequential by design, use ParallelOperator for parallelism
3. Replace ForEach with dynamic ParallelOperator: Transform foreach items into parallel branches at runtime

## Related

Issue #358 acceptance test claimed parallel execution but was actually sequential.

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Edit Issue #359