refactor(query-engine): add inspectable native query DAGs #728

Goal

Make native query execution inspectable by compiling request-specific operator DAGs and logging them, without changing execution semantics.

Implemented

• Internal DAG nodes for store reads, window composition, key resolution, estimation, top-k limiting, and formatting.
• Instant and native range paths compile and debug-log the DAG.
• A concise node reference lives beside the module.
• Existing query execution remains unchanged.

Follow-up

The plan executor is intentionally deferred to issue #729.

Non-goals

• User-authored execution operators in YAML.
• Planner materialization changes.
• Cross-query common-subexpression optimization.
• Replacing the existing executor in this issue.