feat(clickhouse): execute typed relational aggregates

[zzylol]

SQL aggregates above exact relational subtrees were routed through the PromQL summary executor, so original o11y q09/q12 fell back even after their child queries could execute.

This change executes the existing typed ExactOperation::Aggregate over relational rows. Grouping uses typed values with a sort-and-reduce pass; sum/count/min/max/avg share the same execution path. Integer sums are checked, and integer averages accumulate in i128 before conversion to avoid losing cancellation above 2^53. Empty Map metadata uses ClickHouse's Nothing spelling.

Before this PR, an outer SQL SUM over argMax rows could not run in the installed DAG. After this PR, that composition executes locally while its exact leaf stays in ClickHouse. No raw sample store or SQL-specific summary identity is introduced.

Verification: 26 ClickHouse query-engine unit tests pass against the actual pinned sketchlib #139 dependency, including grouped integer sums, overflow, all supported reductions, integer AVG cancellation, and bottom-type metadata. The compiler's four exact-cut tests pass. Native ClickHouse confirms the integer AVG counterexample returns 0.5. Original q07/q09/q12 executed through the external DAG with matching decoded JSON on the final runtime head. q27 executes but has the disclosed one-ULP difference. Independent aggregate source review passed after the AVG correction.

Limitations: nullable numeric aggregate inputs, nonfinite values/results, unsupported intents, and empty min/max remain explicit fallback. Floating reductions can differ by rounding order: q27 differs by one ULP and is not claimed bit-exact. These queries currently have no SDS materialization, so the route is exact_fallback / external_dag, not ASAP acceleration. No performance claim or screenshot applies. Stacked on #648.