expose LocalNet's scan and registry as actual ports

### Problem description

The Java example demonstrating external party creation cannot use the Scan proxy. The problem is that it can't resolve `scan.localhost` against LocalNet supervalidator. The fix is to expose scan and registry in LocalNet using actual ports.

Bernhard has suggested a workaround is to expose the Scan API on a port on straight localhost with a simple edit that adds these two locations to the localhost server in `sv.conf`:
```
location /api/scan {
rewrite ^\/(.*) /$1 break;
proxy_pass http://splice:5012/api/scan;
}
location /registry {
rewrite ^\/(.*) /$1 break;
proxy_pass http://splice:5012/registry;
}
```

The documentation would need to be updated.

Related is https://github.com/digital-asset/cn-quickstart/issues/263

Start with LocalNet's sv.conf and the Java example for external party creation; inspect how Scan and registry are currently exposed. Verify that both endpoints are reachable through actual ports and that the example can resolve them, then update the relevant documentation.

Written by the indexing model from the issue text.