feat: parse into a Document that keeps order and comments

Assisted-by: DeepSeek V4.1 Flash

docs/ARCHITECTURE.md

@@ -43,7 +43,8 @@ Inside the library package, one file owns one concern:
 
 | File | Responsibility |
 |---|---|
-| `parser.go` | The recursive-descent parser. Produces the `map[string]any` tree and enforces the structural rules of TOML 1.1 (table redefinitions, dotted keys, arrays of tables, multi-line inline tables). Reports a 1-based line on failure. |
+| `parser.go` | The recursive-descent parser. Produces the `map[string]any` tree, records the nodes a [Document](API.md#documents) is built from, and enforces the structural rules of TOML 1.1 (table redefinitions, dotted keys, arrays of tables, multi-line inline tables). Reports a 1-based line on failure. |
+| `document.go` | The parsed-document types: `Document`, `Table` and `Entry`, which carry the key order, whether a table was written inline, and the comments. The values they expose are the parser's own tree, not a copy. |
 | `number.go` | Strict numeric tokens: integers in the four radixes with `_` separators, and floats including `inf` and `nan`. Rejects leading zeros, misplaced underscores and malformed fractions. |
 | `datetime.go` | The three local date-time wrapper types and `parseDateTime`, which classifies a token into the four date-time kinds under the strict TOML grammar. |
 | `decode.go` | Maps the parsed tree onto Go values by reflection: struct fields, maps, slices, scalar conversion with overflow checks, `Unmarshaler` dispatch. |