feat: carry the byte offset and column in SyntaxError

Assisted-by: GLM 5.3 Flash

docs/API.md

@@ -642,20 +642,27 @@ sequenceDiagram
 
 ## Types
 
-### `type SyntaxError struct{ Line int; Msg string }`
+### `type SyntaxError struct{ Line, Offset, Column int; Msg string }`
 
-Describes a document the parser rejected, with the 1-based `Line` at which it
-gave up and `Error()` rendering as `interpres: line N: msg`. A malformed
-document is the usual cause; the nesting limit and an input that is not valid
-UTF-8 report through the same type. Read the structured fields with a type
-assertion or `errors.AsType`:
+Describes a document the parser rejected: the 1-based `Line` at which it gave
+up, the `Offset` in bytes the scan stopped at, the 1-based `Column` on that
+line, and `Error()` rendering as `interpres: line N: msg`. A malformed document
+is the usual cause; the nesting limit and an input that is not valid UTF-8
+report through the same type, with the UTF-8 message naming the offset of the
+first invalid byte. Read the structured fields with a type assertion or
+`errors.AsType`:
 
 ```go
 if se, ok := errors.AsType[*interpres.SyntaxError](err); ok {
-	fmt.Println(se.Line, se.Msg)
+	fmt.Println(se.Line, se.Offset, se.Column, se.Msg)
+	fmt.Println(se.SourceLine(data)) // the line, with a caret under Offset
 }
 ```
 
+`SourceLine(src)` renders the source line the error points at from `src`,
+followed by a caret line marking the column, for messages the reader sees
+under the input.
+
 ### `type DecodeError struct{ Path []string; Err error }`
 
 Wraps a decoding failure with the key path at which it happened. `Path` lists