Add strict custom field registry

toolkit/internal/gsp/project.go

@@ -28,6 +28,7 @@ func (p *Project) Validate(stage string) Report {
 			report.addError("unsupported_gsp_version", "", p.Manifest.File, fmt.Sprintf("GSP version %q is not supported by this toolkit", p.Manifest.GSPVersion))
 		}
 	}
+	p.validateFieldRegistry(&report)
 	for _, unit := range p.Units {
 		if unit.ID == "" {
 			report.addError("missing_id", "", unit.File, "GSP requires id")
@@ -66,6 +67,7 @@ func (p *Project) Validate(stage string) Report {
 		if unit.Context == "" {
 			report.addNotice("placeholder", unit.ID, unit.File, "GSP has no context and is treated as placeholder")
 		}
+		p.validateCustomFields(&report, unit)
 	}
 	for id, units := range p.Duplicates {
 		files := make([]string, 0, len(units))