refactor(query-engine): execute native query DAGs

[milindsrivastava1997]

Goal

Make the native query DAG the source of execution rather than debug-only metadata.

Scope

• Add a plan executor that consumes the DAG introduced in refactor(query-engine): add inspectable native query DAGs #728.
• Move existing range execution phases behind DAG nodes without changing query results.
• Preserve instant/range equivalence, sliding exact-cover semantics, DeltaSet key replay, top-k behavior, and fallback behavior.
• Add equivalence and failure-path regression tests for the executor cutover.

Non-goals

• Redesigning planner materialization or user YAML.
• Cross-query optimization.

Acceptance criteria

• Native leaf execution runs through the plan executor.
• Existing native and e2e correctness suites remain green.
• Invalid plans produce actionable errors rather than silently falling back.