Expose validation errors through the root API

internal/apicheck/main_test.go

@@ -101,6 +101,35 @@ func Open(context.Context) *Handle { return nil }
 	}
 }
 
+func TestValidationDiagnosticTypesBelongToFacade(t *testing.T) {
+	valid := `package facade
+type ValidationCheck string
+type ValidationFinding struct{}
+type FileSizeMeasurement struct{}
+type ValidationError struct {
+	Check ValidationCheck
+	Findings []ValidationFinding
+	Measurement *FileSizeMeasurement
+	Cause error
+}
+`
+	pkg := typecheck(t, token.NewFileSet(), modulePath, valid)
+	if violations := checkValidationTypes(pkg); len(violations) != 0 {
+		t.Fatalf("checkValidationTypes() = %v, want facade-owned diagnostics", violations)
+	}
+
+	implementation := typecheck(t, token.NewFileSet(), "example.com/implementation", `package implementation
+type State string
+`)
+	fset := token.NewFileSet()
+	fixtureImporter := singlePackageImporter{path: "example.com/implementation", pkg: implementation}
+	invalid := strings.Replace(valid, "type ValidationCheck string", "import \"example.com/implementation\"\ntype ValidationCheck = implementation.State", 1)
+	pkg = typecheckWithImporter(t, fset, modulePath, invalid, fixtureImporter)
+	if violations := checkValidationTypes(pkg); !strings.Contains(strings.Join(violations, "\n"), "ValidationCheck is not a facade-owned named type") {
+		t.Fatalf("checkValidationTypes() = %v, want ownership violation", violations)
+	}
+}
+
 func TestActualRepositoryBoundaries(t *testing.T) {
 	if err := checkRepository(t.Context(), "../.."); err != nil {
 		t.Fatal(err)