fix(encode): keep a []any of tables in the value-array form

Assisted-by: GLM 5.3

docs/API.md

@@ -331,6 +331,11 @@ tree, _ := interpres.Parse([]byte(`arr = [1, {a = 2}, "x"]`))
 out, _ := interpres.Marshal(tree) // arr = [1, {a = 2}, "x"]
 ```
 
+A `[]any` holding only tables keeps the value-array form as well, because that
+is the shape `Parse` gives a value array of inline tables; emitting it as
+`[[headers]]` would re-parse as `[]map[string]any` and change the value's type
+across a round-trip.
+
 ### Empty arrays
 
 A nil slice is always omitted. An empty (length 0) array of tables is always