catchall issue for miscellaneous differences in contextual lookups

There are a bunch of fonts with some differences here, and teasing apart the different causes will be most of the work in fixing them, so this is a general issue for marking those fonts, e.g:

```
python3 -m ttx_diff 'https://github.com/notofonts/grantha?6bb911fa06#sources/NotoSansGrantha.glyphs'
```

Start by running the provided python3 -m ttx_diff command against the NotoSansGrantha source and inspect the reported contextual lookup differences. Trace the differences to their separate causes, then verify that the relevant fonts no longer show those discrepancies.

Written by the indexing model from the issue text.