Function encode_base_size 

pub fn encode_base_size(b: &BaseSize) -> String

Canonical string encoding of a base size, shared by the conformance runner and its tests so both sides of the cross-impl contract agree byte-for-byte. Mirrors the TS encodeBase. Examples: round 32 → "round:32"; oval 75×42 → "oval:75x42"; small flyer → "flying-base:small:draft"; hull → "hull:draft".