Runtime: tests for the native lowering of PhysicalASAPDAG #662

Found by the third review of #541.

Problem

The runtime compiler's native lowering (physical_planner, formerly unified_physical_planner) has almost no tests of its own. Until #541 removed the PromQL fallback evaluator, nearly every runtime test went through that evaluator. Untested:

• SummaryAgg per entity, FinalizeExactAccumulator (Int64 → Float64), MaintainPopulation / EvaluatePopulation;
• the ingestion-time aligned binary and the query-time series_binary;
• compile_candidate, cut_candidate, enumerate_frontiers, frontier_from_timing, select_candidate;
• bind_with_data_sources and the in-memory source;
• the LOWERED_NODES counter exists for compile-once tests but nothing reads it.

An end-to-end test of a maintained population, or of a summary over a transform, would have caught the ingestion-time routing bug the review found.

Proposal

One test per native operator family that builds a small timed PhysicalASAPDAG, compiles it, runs it on in-memory data and checks the rows; plus tests for the frontier / candidate functions and bind_with_data_sources.