refactor: unify the error paths in one Path type

Assisted-by: GLM 5.3 Flash

docs/API.md

@@ -738,26 +738,32 @@ if se, ok := errors.AsType[*interpres.SyntaxError](err); ok {
 followed by a caret line marking the column, for messages the reader sees
 under the input.
 
-### `type DecodeError struct{ Path []string; Err error }`
+### `type DecodeError struct{ Path Path; Err error }`
 
 Wraps a decoding failure with the key path at which it happened. `Path` lists
 one segment per level from the document root, the outermost key first: a key
 contributes its name, an array element its bracketed index, so the path of the
-`weight` field in the first item reads `["items", "[0]", "weight"]`. The
-rendered message is unchanged by the type; read the fields instead of parsing
-the message:
+`weight` field in the first item reads `["items", "[0]", "weight"]` and its
+`String()` renders `items[0].weight`. Read the fields instead of parsing the
+message:
 
 ```go
 if de, ok := errors.AsType[*interpres.DecodeError](err); ok {
-	fmt.Println(de.Path, de.Err)
+	fmt.Println(de.Path.String(), de.Err)
 }
 ```
 
-### `type EncodeError struct{ Path string; Err error }`
+### `type EncodeError struct{ Path Path; Err error }`
 
-Wraps an encoding failure with the key path of the value that failed, in the
-document's own notation: `server.ports[2]`. Read it with `errors.AsType` the
-same way.
+Wraps an encoding failure with the key path of the value that failed, the
+same `Path` type the decode error carries, so `server.ports[2]` reads the
+same on both sides. Read it with `errors.AsType` the same way.
+
+### `type Path []string`
+
+The path both error wrappers carry, one segment per level from the document
+root. `String()` renders the TOML notation: keys join with dots, an index
+attaches to the previous segment in brackets, `items[0].weight`.
 
 ### `type Decoder`