Function sign_definition_from_wit 

pub fn sign_definition_from_wit(name: String, spec: SignSpec) -> SignDefinition

Convert a WIT sign-spec plus its namespaced name to the native definition.

Total — every WIT shape maps, so there is no failure mode here and define-sign’s err is reserved for identity-level rejections. The glyph is NOT truncated here: SignDefinition::glyph_char does that at paint time, so a future gutter that can afford a wider cell does not need this conversion changed, and :describe-sign can still show what the plugin actually declared.