feat: add the Document edit pipeline with comment-preserving write

Assisted-by: GLM 5.3 Flash

docs/API.md

@@ -150,6 +150,37 @@ document and from `doc.Map()` is the same value.
 array of tables, and the inline tables inside a value array, with `nil` for the
 elements that are not tables.
 
+### Editing a document
+
+The document is writable, which makes the read-change-write loop a round trip
+through one value. The typed getters read with one call:
+
+| Getter | Returns |
+|---|---|
+| `GetString(key)` | `(string, bool)` |
+| `GetInt(key)` | `(int64, bool)` |
+| `GetFloat(key)` | `(float64, bool)` |
+| `GetBool(key)` | `(bool, bool)` |
+| `GetArray(key)` | `([]any, bool)` |
+| `GetTable(key)` | `(*Table, bool)` |
+
+`Set(key, value)` stores a value, keeping an existing key's position and
+comments and appending a new key to the end; a `map[string]any` value becomes
+a table of its own under a header, its keys in sorted order. `Delete(key)`
+removes a key and everything it holds. Every method exists on `Document` for
+the root table and on `Table` for the table itself.
+
+`Marshal` writes the document back as it stands: keys in written order, the
+comments above the lines and headers they belonged to, tables that were
+written inline written inline again. `UnmarshalDocument(doc, v)` decodes the
+edited document into a typed destination without parsing again.
+
+```go
+doc, err := interpres.Parse(data)
+doc.Set("port", 9090)
+out, err := interpres.Marshal(doc)
+```
+
 ### Comments
 
 A comment belongs to the line it precedes or follows, and to the node that line
@@ -868,8 +899,10 @@ long as no setter races with a call.
 
 ### `type Document`, `type Table`, `type Entry`
 
-See [Documents](#documents). A `Document` is what `Parse` returns, and it is
-not a value `Marshal` accepts.
+See [Documents](#documents). A `Document` is what `Parse` returns, and
+`Marshal` writes it back: the keys in written order, the comments in place,
+the inline tables inline. `UnmarshalDocument(doc, v)` decodes it without
+parsing again.
 
 ### `type Marshaler interface{ MarshalTOML() (any, error) }`