feat: add OffsetDateTime, nesting limits and uniform Marshaler dispatch

Assisted-by: DeepSeek V4.1 Flash

decode.go

@@ -95,12 +95,10 @@ func (d *decoder) assign(data any, dst reflect.Value) error {
 		return setInt(dst, v)
 	case float64:
 		return setFloat(dst, v)
+	case OffsetDateTime:
+		return setOffsetDateTime(v, dst)
 	case time.Time:
-		if dst.Type() != timeType {
-			return fmt.Errorf("interpres: cannot assign datetime to %s", dst.Type())
-		}
-		dst.Set(reflect.ValueOf(v))
-		return nil
+		return setDateTime(v, dst)
 	default:
 		rv := reflect.ValueOf(data)
 		if rv.IsValid() && dst.Type() == rv.Type() {
@@ -235,6 +233,36 @@ func (d *decoder) assignTableSlice(items []map[string]any, dst reflect.Value) er
 
 // --- low-level setters -----------------------------------------------------
 
+// setOffsetDateTime stores an offset date-time: in a wrapper destination as it
+// is, and in a plain time.Time, which takes the instant with the offset the
+// document wrote, so a timestamp field does not have to name the wrapper.
+func setOffsetDateTime(v OffsetDateTime, dst reflect.Value) error {
+	switch dst.Type() {
+	case offsetDateTimeType:
+		dst.Set(reflect.ValueOf(v))
+	case timeType:
+		dst.Set(reflect.ValueOf(v.Time))
+	default:
+		return fmt.Errorf("interpres: cannot assign datetime to %s", dst.Type())
+	}
+	return nil
+}
+
+// setDateTime stores a time.Time that reached the tree directly, which is the
+// shape a tree built by hand carries. Dates the parser produced arrive as
+// OffsetDateTime instead.
+func setDateTime(v time.Time, dst reflect.Value) error {
+	switch dst.Type() {
+	case timeType:
+		dst.Set(reflect.ValueOf(v))
+	case offsetDateTimeType:
+		dst.Set(reflect.ValueOf(OffsetDateTime{v}))
+	default:
+		return fmt.Errorf("interpres: cannot assign datetime to %s", dst.Type())
+	}
+	return nil
+}
+
 func setBasic(dst, val reflect.Value, kind string) error {
 	if dst.Kind() != val.Kind() {
 		return fmt.Errorf("interpres: cannot assign %s to %s", kind, dst.Type())