Function gutter_level 

pub fn gutter_level(severity: ErrorSeverity) -> GutterSeverityLevel

CM.3c: map the parser-native [ErrorSeverity] onto the renderer- facing [GutterSeverityLevel] (Error→Error, Warning→Warning, Info→Info, Note→Info). The single conversion in the whole compilation-severity pipeline — the parser, the inbound bus, and the AppEffect payload all carry ErrorSeverity, and the host arm calls this once when writing the render-state slot. Note collapses to Info because GutterSeverityLevel has no Note rank; there is no reverse conversion, so nothing is lost by round-trip.