resolveFileFeatures function
Resolves the file-level FeatureSet for edition with the file's explicit
features overrides applied on top of the edition base.
fileFeatures is the file's options.features (a subset of the six keys,
string-valued); null/absent leaves the base unchanged. Overriding a
feature that is FIXED at edition throws ArgumentError.
Implementation
Map<String, String> resolveFileFeatures(
int edition,
Map<String, Object?>? fileFeatures,
) {
validateEditionInRange(edition);
final base = baseFeaturesForEdition(edition);
_applyOverrides(base, fileFeatures, edition);
return base;
}