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

Assisted-by: GLM 5.3 Flash

docs/API.md

@@ -196,6 +196,8 @@ table with `Number`, which keeps the literal; see
 - `*struct`, matched per the field rules below
 - `*map[string]any` or `*map[string]T`, keys become map keys and values decode
   into `T` recursively
+- `*OrderedMap`, the keys fill in the order the document wrote them; see
+  [Ordered tables](#ordered-tables)
 - `*any`, receives the whole parsed tree unchanged
 
 Anything else returns `interpres: decode target must be a non-nil pointer`.
@@ -366,6 +368,26 @@ and `from_text = "1h30m"` decode to the same duration. Text that
 `time.ParseDuration` rejects, `d = "90"` among it, fails with
 `interpres: invalid duration "90"`.
 
+### Ordered tables
+
+`OrderedMap` is a string-keyed table that remembers the order its keys were
+set in, the shape a `map[string]any` cannot carry. Decoding into one fills it
+in the order the document wrote the keys, and `Marshal` writes one back in
+that order, where a map destination carries no order and a map source sorts
+its keys. The type is a decode target on its own, in a struct field, and as
+the element of an array of tables.
+
+```go
+var cfg OrderedMap
+err := interpres.Unmarshal(data, &cfg)
+out, err := interpres.Marshal(&cfg) // the keys come back in written order
+```
+
+The values are untyped, the shape the parser produces, so a nested table
+inside an `OrderedMap` is a plain `map[string]any`; the order is kept at the
+level the `OrderedMap` sits at. Inside a value array an `OrderedMap` renders
+as an ordinary inline table, whose keys are sorted.
+
 ### Strict decoding
 
 By default unknown keys are dropped silently. A `Decoder` built with
@@ -782,6 +804,11 @@ The literal a number was written with, what `UseNumber` decodes into and what
 `Marshal` writes back as it is. See
 [Numbers as literals](#numbers-as-literals).
 
+### `type OrderedMap`
+
+The string-keyed table that keeps its key order on both the encode and the
+decode side. See [Ordered tables](#ordered-tables).
+
 ### Date-time wrappers
 
 ```go