1996 Missing in unit lookup, converter

As in the following error:

```
F20260416 23:20:45.238920 53936 uint8_lookup.cc:601] Check failed: it != lookup_table->end() Could not find 1996
```

Start by tracing the unit lookup and converter path associated with the failure at uint8_lookup.cc:601, then determine why lookup_table lacks 1996. Reproduce the reported error and inspect the surrounding lookup code; done means the converter handles unit 1996 without the lookup failure and the relevant test or reproduction passes.

Written by the indexing model from the issue text.