Remove optional offset and fallback, guarantee offset by construction

ParserError.Offset is now a plain exported int field, always set:
- The parser sets it via setErrOffset() when capturing parse errors
- strict.go sets it from the key's Raw range at construction
- wrapDecodeError computes it inline from cap(document) - cap(highlight)

This eliminates:
- The SetOffset/Offset() accessor methods and offsetValid flag
- The subsliceOffset fallback function in errors.go
- Any conditional logic around whether the offset is present

The offset is guaranteed by construction at every path that creates
or consumes a ParserError.

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

errors.go

@@ -99,10 +99,7 @@ func (e *DecodeError) Key() Key {
 //
 //nolint:funlen
 func wrapDecodeError(document []byte, de *unstable.ParserError) *DecodeError {
-	offset, ok := de.Offset()
-	if !ok {
-		offset = subsliceOffset(document, de.Highlight)
-	}
+	offset := cap(document) - cap(de.Highlight)
 
 	errMessage := de.Error()
 	errLine, errColumn := positionAtEnd(document[:offset])
@@ -264,9 +261,3 @@ func positionAtEnd(b []byte) (row int, column int) {
 
 	return row, column
 }
-
-// subsliceOffset returns the byte offset of subslice within data.
-// subslice must share the same backing array as data.
-func subsliceOffset(data []byte, subslice []byte) int {
-	return cap(data) - cap(subslice)
-}