Refactor aggregation into classes

[j6k4m8]

Refactors COUNT, SUM, AVG, MAX, and MIN into concrete aggregation classes with shared grouping logic while preserving existing query semantics. Removes aggregation-related executor state mutation and adds coverage for grouping, aliases, ordering, multiple functions, null behavior, and repeated query execution.

continues pulling latest from #97 into master :)

[codspeed-hq]

Merging this PR will degrade performance by 10.98%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

❌ 2 regressed benchmarks
✅ 97 untouched benchmarks

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
❌	test_nested_nots_in_statements[MultiDiGraph]	60.7 ms	68.2 ms	-10.99%
❌	test_nested_nots_in_statements[DiGraph]	60.7 ms	68.2 ms	-10.97%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing aggregation-classes (c75cfde) with master (3da4c13)¹}

Footnotes

1. No successful run was found on master (e04db72) during the generation of this report, so 3da4c13 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩