RFC: L2 gate simplification (core vs full L2)

[drozhzhin-n-e]

Synthesis notes ~17 L2 gates; most starters score 650-780 but fail gates. v1.4 documents core vs full in l2-gates.md. v2.0 RFC: consider normative core L2 subset or deferring selected gates to L3. See docs/rfc/README.md and docs/l2-gates.md.