Add aggregate and grouping selection support (#312)

[salmonumbrella]

Fixes #312

• What changed: Added count(...) and count-distinct(...) selection parsing, validated exposed intermediary variables, and normalized aggregate variable syntax before generating Datalog aggregates.
• Why this fixes it: aggregate/grouping selections now compile and return stable counts for intermediary nodes as documented.

[coderabbitai]

Important

Review skipped

Auto reviews are disabled on this repository. Please check the settings in the CodeRabbit UI or the .coderabbit.yaml file in this repository. To trigger a single review, invoke the @coderabbitai review command.

You can disable this status message by setting the reviews.review_status to false in the CodeRabbit configuration file.

Use the checkbox below for a quick retry:

• 🔍 Trigger review

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands and usage tips.}