[P1] Binary min/max return () — only the collection form works

[ecto]

[min 3 7] and [max 3 7] return (). Only the collection form
works:

[min #[3 1 2]]   ; 1
[min 3 7]        ; ()  -- expected 3

This is surprising because every other functional language ships
binary min/max as the primary form.

Acceptance criteria

• [min a b] returns the smaller of two comparable values.
• [max a b] returns the larger.
• Existing collection form keeps working: [min xs] / [max xs].
• Tests for both arities, mixed Int/Float, and strings (if ordered
  comparisons are intended to work on them).