`basic_pbf_builder<TBuffer, T>` submessage constructor does not enforce correct parent tag type.

Nobody has claimed this yet.

This constructor of `basic_pbf_builder`:

```cpp
/**
* Construct a pbf_builder for a submessage from the pbf_message or
* pbf_writer of the parent message.
*
* @param parent_writer The parent pbf_message or pbf_writer
* @param tag Tag of the field that will be written
*/
template
basic_pbf_builder(basic_pbf_writer& parent_writer, P tag) :
basic_pbf_writer{parent_writer, pbf_tag_type(tag)} {
}
```

Does not enforce that `P` is the correct tag type of `parent_writer`.

Would it be acceptable to submit a PR that replaces or overloads this constructor with:

```cpp
/**
* Construct a pbf_builder for a submessage from the pbf_builder of the parent message.
*
* @param parent_writer The parent pbf_builder
* @param tag Tag of the field that will be written
*/
template
basic_pbf_builder(basic_pbf_builder& parent_builder, P tag) :
basic_pbf_writer{parent_builder, pbf_tag_type(tag)} {
}
```

I did not verify that this compiles, but I think it explains the idea.

Start at the basic_pbf_builder submessage constructor shown in the issue and inspect the related basic_pbf_writer, pbf_tag_type, and tag-type definitions. Verify how parent and tag types are represented, then add coverage showing that valid construction remains possible and an incorrect parent tag type is rejected at compile time.

Written by the indexing model from the issue text.