feat: add Decoder.UseNumber and the Number type

Assisted-by: GLM 5.3 Flash

decode.go

@@ -26,6 +26,7 @@ var timeType = reflect.TypeFor[time.Time]()
 var (
 	unmarshalerType     = reflect.TypeFor[Unmarshaler]()
 	textUnmarshalerType = reflect.TypeFor[encoding.TextUnmarshaler]()
+	numberType          = reflect.TypeFor[Number]()
 )
 
 // The per-type flags record which interface lookups a decode into that type
@@ -181,6 +182,8 @@ func (d *decoder) assign(data any, dst reflect.Value) error {
 			return setDuration(dst, v)
 		}
 		return setBasic(dst, reflect.ValueOf(v), "string")
+	case Number:
+		return setNumber(dst, v)
 	case bool:
 		return setBasic(dst, reflect.ValueOf(v), "bool")
 	case int64:
@@ -389,6 +392,28 @@ func setDuration(dst reflect.Value, s string) error {
 	return nil
 }
 
+// setNumber stores a Number, the literal UseNumber keeps. A Number destination
+// takes the literal as it is; every other destination takes the evaluated
+// value through the ordinary rules, so an integer field, a float field and a
+// duration field all read a Number the way they read the evaluated kind.
+func setNumber(dst reflect.Value, n Number) error {
+	if dst.Type() == numberType {
+		dst.SetString(string(n))
+		return nil
+	}
+	v, err := decodeNumber(string(n))
+	if err != nil {
+		return fmt.Errorf("interpres: %w", err)
+	}
+	switch v := v.(type) {
+	case int64:
+		return setInt(dst, v)
+	case float64:
+		return setFloat(dst, v)
+	}
+	return fmt.Errorf("interpres: cannot assign number to %s", dst.Type())
+}
+
 func setInt(dst reflect.Value, v int64) error {
 	switch dst.Kind() {
 	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64: