syntaxToEdition function
Maps the legacy syntax keyword to its edition sentinel.
An empty or "proto2" syntax is proto2; "proto3" is proto3. Used by the
legacy-inference path so proto2/proto3 descriptors resolve through the same
edition-defaults table as editions files.
Implementation
int syntaxToEdition(String syntax) {
if (syntax == 'proto3') return editionProto3;
return editionProto2;
}