Cache error offset in ParserError for safer position tracking

Instead of requiring downstream consumers to re-derive the byte offset
from pointer arithmetic on the Highlight slice, compute and cache the
offset inside the parser at error-capture time via setErrOffset().

This is safer because:
- The parser is the one place where the backing-array guarantee is known
  to hold (Highlight is always a subslice of the parse buffer)
- Downstream consumers (wrapDecodeError) can use the cached offset
  directly, avoiding the need for pointer comparison
- Errors created outside the parser (strict.go) set the offset from
  existing Raw ranges, which are already correct by construction

Add ParserError.SetOffset/Offset methods for setting and retrieving the
cached offset. Update wrapDecodeError to prefer the cached offset when
available, falling back to subsliceOffset for backward compatibility.

Co-authored-by: Thomas Pelletier <thomas@pelletier.dev>

errors.go

@@ -100,7 +100,12 @@ func (e *DecodeError) Key() Key {
 //
 //nolint:funlen
 func wrapDecodeError(document []byte, de *unstable.ParserError) *DecodeError {
-	offset := subsliceOffset(document, de.Highlight)
+	var offset int
+	if o, ok := de.Offset(); ok {
+		offset = o
+	} else {
+		offset = subsliceOffset(document, de.Highlight)
+	}
 
 	errMessage := de.Error()
 	errLine, errColumn := positionAtEnd(document[:offset])