Consider secondary implementation with token injection

[P3294](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p3294r2.html#type-erasure) has a motivating example that looks very similar to what `protocol` is attempting to achieve (see the "Type Erasure" section). As far as I am aware, it is currently the most popular proposal for generating code in C++29 and has some functioning implementations already.

If it helps with standardization, we could pursue a secondary C++29 implementation using token injection.

No specific file or test is named. Start by reading the Type Erasure section of P3294 and comparing it with the current protocol implementation; done would mean reaching a project decision about whether to pursue a secondary C++29 implementation using token injection.

Written by the indexing model from the issue text.