refactor(query-engine): execute complete native-query DAGs

[milindsrivastava1997]

Goal

Make the native query DAG the authoritative execution representation for complete native PromQL queries, not only individual native leaves.

Depends on

• refactor(query-engine): execute native query DAGs #729: native leaf DAG execution.

Scope

• Compile complete supported native queries into one request-specific DAG.
• Represent scalar constants and binary arithmetic in the DAG.
• Execute each leaf through the DAG and perform label matching, timestamp alignment, and arithmetic through DAG nodes.
• Preserve existing PromQL output-label and fallback behavior.
• Cover the work with end-to-end tests through handle_query_promql and handle_range_query_promql, real precompute ingestion fixtures, and the PromQL compliance suite.

Non-goals

• Planner materialization redesign or user-YAML changes.
• Cross-query optimization.
• Changing supported PromQL semantics.

Acceptance criteria

• Supported scalar and vector-vector binary native queries execute through one complete DAG.
• Instant and range results preserve existing label matching and timestamp-alignment behavior.
• Unsupported or invalid complete DAGs fail with actionable errors rather than silently falling back.
• Native E2E correctness and PromQL differential suites remain green.