feat(promql): compile and execute count/avg aggregation families from the ASAPQuery matrix #503

Audit evidence

The PR-derived ASAPQuery PromQL matrix includes count, avg, count_over_time, and avg_over_time (especially ASAPQuery #700). Against current main, the backend-local physical compiler:

• compiles count_over_time(m[1m]);
• panics in stage config emission for count(m) because the selected sketch algorithm has no backend mapping;
• rejects avg(m) and avg_over_time(m[1m]) because no executable Collector lifecycle is selected.

This is a warm-path capability gap, not a parser gap. Parent matrix: #501.

Required behavior

• Compile and execute all four shapes for instant and range requests, with optional by (...) where ASAPQuery supports it.
• Represent avg as a semantics-preserving sum/count composition and retain both materialization/readout dependencies.
• Map count to an executable exact or sketch family without an unreachable! panic; unknown families must fail closed.
• Preserve labels, float result type, timestamps, empty-window behavior, and Prometheus NaN/divide-by-zero semantics.

Acceptance tests

Add these cases to the backend process differential E2E matrix, assert parity with Prometheus, and assert data_source: warm for both /api/v1/query and /api/v1/query_range.