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>

unstable/parser_test.go

@@ -766,7 +766,7 @@ func TestErrorHighlightPositions(t *testing.T) {
 	}
 }
 
-func TestParserError_CachedOffset(t *testing.T) {
+func TestParserError_Offset(t *testing.T) {
 	examples := []struct {
 		desc       string
 		input      string
@@ -803,12 +803,8 @@ func TestParserError_CachedOffset(t *testing.T) {
 			if !errors.As(err, &perr) {
 				t.Fatalf("expected ParserError, got %T", err)
 			}
-			offset, ok := perr.Offset()
-			if !ok {
-				t.Fatal("expected offset to be set")
-			}
-			if offset != e.wantOffset {
-				t.Errorf("cached offset: got %d, want %d", offset, e.wantOffset)
+			if perr.Offset != e.wantOffset {
+				t.Errorf("offset: got %d, want %d", perr.Offset, e.wantOffset)
 			}
 		})
 	}