feat: aggregates, GROUP BY/HAVING, expression type-checking, and CI

[Aybavs]

Adds M3 to the engine.

Aggregates

• Parse GROUP BY, HAVING, and aggregate calls (COUNT, SUM, AVG, MIN, MAX)
• New Aggregate operator with a group-key encoder
• Planner lowers aggregate expressions and plans grouped queries end to end
• Correct NULL semantics: AVG over an all-NULL group is NULL, COUNT ignores NULLs

Correctness fixes found while building this

• Type-check planned expressions, so type errors surface before execution rather than producing garbage at runtime
• Compare large numeric values exactly instead of routing everything through float64, which lost precision for big integers

Infrastructure

• Minimal GitHub Actions workflow running vet and race tests

All packages pass go vet ./... and go test -race ./....

Closes #32, closes #33, closes #34, closes #35, closes #36, closes #37, closes #38