tunnel back to parameterised knotname crashes web export

the following

```
-> tunnel(-> knot) -> END

=== knot
yo.
-> DONE
=== tunnel (-> go )
->-> go
```

runs in inky as expected, but crashes when compiling to a playable web version with an "unknown error". Removing the

`->-> (parametername)`

line removes the error.

Reproduce the supplied ink example in Inky, then compare playable web export with and without the parameterised tunnel line. Trace the web-export compilation path from that minimal case; done when the example exports without the unknown error while retaining the parameterised tunnel.

Written by the indexing model from the issue text.