perf(pact_matching): Eliminate redundant tree-cloning in V2 engine interpreter

Change walk_tree and all execute_* methods to take ExecutionPlanNode by value
instead of by reference. The interpreter now moves/transforms the plan tree
into the executed tree rather than cloning it.

Key changes:
- walk_tree: &ExecutionPlanNode -> ExecutionPlanNode; CONTAINER and PIPELINE
  branches use children.into_iter() instead of VecDeque::from(children.clone()),
  and push(child_result) instead of push(child_result.clone())
- execute_action and all ~25 execute_* methods: owned node, destructure at top
- validate_one/two/three_args and validate_args: take Vec<ExecutionPlanNode>
  directly, use into_iter() to move children to walk_tree
- evaluate_children: takes children: Vec<ExecutionPlanNode> by value
- execute_plan: clones plan_root once at entry (public API unchanged)

Benchmark results (execute phase):
  text body:        -30%
  JSON equality:    -35%
  JSON with rules:  -33%

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: rholshausen