docs: define DTQL aggregation semantics

[trakhimenok]

Summary

• define the YAML DTQL groupBy and aggregation contract
• document aggregate and binary expressions, aliases, HAVING, ordering, offsets, and limits
• specify native-provider and generic client-side execution semantics

This is stacked on PR #1 for negative projection and documents the contract implemented by dal-go/dalgo#174 and dal-go/dalgo2sql#187.

Verification

• git diff --check
• implementation and documentation were reviewed together across all three repositories

[trakhimenok]

Review by unknown@codex@unknown (head 9cc46c62a732)
Reviewed-Head: 9cc46c6

Reviewed the exact current documentation patch against the merged DTQL base. It defines YAML aggregation syntax, validation, execution order, value semantics, resource limits, and explicit non-goals consistently with the implemented DALgo and DALgo2SQL behavior.