feat: add OrderedMap, the table that keeps its key order

Assisted-by: GLM 5.3 Flash

decode.go

@@ -18,9 +18,13 @@ import (
 // decoder maps a parsed TOML tree onto Go values via reflection. ctx is the
 // context a cancellable entry point handed in, and reaches an
 // UnmarshalerContext destination; entry points without one leave it nil.
+// nodes is the document's node index, present only when a destination can
+// reach an OrderedMap and the parse built the tree its key order is read
+// from.
 type decoder struct {
 	disallowUnknown bool
 	ctx             context.Context
+	nodes           nodeIndex
 }
 
 func newDecoder() *decoder { return &decoder{} }
@@ -273,6 +277,9 @@ func textUnmarshalerOf(dst reflect.Value) (encoding.TextUnmarshaler, bool) {
 }
 
 func (d *decoder) assignTable(tbl map[string]any, dst reflect.Value) error {
+	if dst.Type() == orderedMapType {
+		return d.fillOrderedMap(tbl, dst)
+	}
 	switch dst.Kind() {
 	case reflect.Struct:
 		return d.assignStruct(tbl, dst)