feat: add ParseFile and Valid

Assisted-by: GLM 5.3 Flash

docs/API.md

@@ -50,6 +50,29 @@ tree, err := interpres.ParseMap([]byte("title = \"x\"\nport = 8080\n"))
 
 The cancellable variant of `ParseMap`.
 
+### `func ParseFile(path string) (*Document, error)`
+
+Reads the file at `path` and parses it into a [Document](#documents), the shape
+`Parse` gives. Both a read failure and a parse failure come back with the file
+name as their first words, wrapped so `errors.AsType` still reaches the
+`SyntaxError` inside a parse failure.
+
+```go
+doc, err := interpres.ParseFile("config.toml")
+```
+
+### `func Valid(data []byte) error`
+
+Reports whether `data` is a valid TOML document: `nil` when the parser accepts
+it, the parse error when it does not. It is the library call the `-validate`
+mode of interpres-decode is built on.
+
+```go
+if err := interpres.Valid(data); err != nil {
+	fmt.Println("invalid:", err)
+}
+```
+
 ## Documents
 
 `Parse` returns a `Document`: the value tree together with what a map cannot