feat: add Decoder.UseNumber and the Number type

Assisted-by: GLM 5.3 Flash

encode.go

@@ -750,6 +750,11 @@ func normaliseValue(v reflect.Value) (any, error) {
 	if v.Type() == durationType {
 		return time.Duration(v.Int()).String(), nil
 	}
+	// A Number goes out as the literal it carries, the one string-kind value
+	// that is not written quoted.
+	if v.Type() == numberType {
+		return Number(v.String()), nil
+	}
 	// A type that renders itself as text becomes a TOML string, scalar kinds
 	// and structs alike.
 	s, isText, err := textValue(v)
@@ -1171,6 +1176,12 @@ func (e *encoder) writeValue(val any) error {
 	case int64:
 		e.buf.WriteString(strconv.FormatInt(v, 10))
 		return nil
+	case Number:
+		if _, err := decodeNumber(string(v)); err != nil {
+			return fmt.Errorf("interpres: %w", err)
+		}
+		e.buf.WriteString(string(v))
+		return nil
 	case float64:
 		return e.writeFloat(v)
 	case time.Time: