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

[milindsrivastava1997]

Summary

Adds an internal, request-specific DAG for native accelerated queries and emits it at debug level for instant and range execution paths.

The existing range executor remains unchanged. Executing DAGs is deferred to #729.

DAG nodes

• StoreRead: no inputs; fetches one aggregation over a time range.
• ComposeWindows: one store-read input; composes buckets for each output timestamp.
• ResolveKeys: value input plus optional keys input; resolves output keys.
• Estimate: resolved-keys input; computes the requested statistic.
• LimitTopK: estimate input; retains the highest-ranked k candidates.
• Format: estimate or top-k input; applies protocol-specific output labels.

Verification

• DAG compiler tests cover key/value fan-in, all range output timestamps, top-k root ordering, and invalid top-k metadata.
• Native range regression suite: 26 passing tests.
• Commit hooks: formatting, Cargo check, clippy, and tests passed.

Closes #728.