- Add ParallelSubplanScheduler class for managing parallel subplan execution
- Implement SubplanQueue for tracking pending, active, and completed subplans
- Implement SchedulerState for immutable scheduler state tracking
- Support configurable max_parallel concurrency limit (1-50)
- Support sequential, parallel, and dependency-ordered execution modes
- Automatic queuing of subplans when max_parallel limit is reached
- Parent plan blocks until all subplans complete
- Comprehensive failure handling and retry logic
- Merge strategy selection for combining subplan outputs
- Add comprehensive BDD test suite with 50+ scenarios
- Test coverage for concurrency control, queue management, and state tracking

@@ -0,0 +435,4 @@
+    """Try to create scheduler with None config."""
+    try:
+        ParallelSubplanScheduler(
+            config=None,  # type: ignore

@@ -0,0 +476,4 @@
+    assert context.result.all_succeeded is True
+
+
+@then("the peak concurrent execution should not exceed {max_parallel:d}")

@@ -0,0 +73,4 @@
+        """Check if all subplans have completed."""
+        return len(self.pending) == 0 and len(self.active) == 0
+
+    def move_to_active(self, count: int) -> SubplanQueue:

@@ -0,0 +207,4 @@
+        """Current execution mode."""
+        return self._config.execution_mode
+
+    def schedule(