Meta bug context: Generics and new style proc in my toy projects

A little bit of context for my recent discovery of various generics issues, often in generic structs and new stlye procs (there: also IR lowering issues). I realize that I am tickling some still some work-in-progress parts of dslx :)

* #4909 (<- this in particular seems pretty load-bearing to be fixed for most of the other ones)
* #4901
* #4895
* #4920 (was holding it wrong)
* #4924
* #4925
* #4926
* #4898
* #4894
* already fixed #4879

Since it sometimes help to see the wider context, I was discovering these playing around with some toy projects, so here you see the source code I was playing with:

* https://github.com/hzeller/xls-convolve in particular the [ringbuffer.x](https://github.com/hzeller/xls-convolve/blob/main/ringbuffer.x)
* https://github.com/hzeller/difference-engine and in there the XLS implementation [iterative_polynomial_sampler.x](https://github.com/hzeller/difference-engine/blob/main/iterative_polynomial_sampler.x)

For some of the issues, I have provided root-cause analysis and a potential patch; but since I didn't dig into it myself but let `antigravity` do it, these are merely unverified suggestions, so I leave it to you to verify and provide a 'proper' fix (having said that, things work more smoothly with the bunch of patches applied)

Review the linked issues as a group, starting with #4909, which the report identifies as load-bearing, then inspect the toy sources ringbuffer.x and iterative_polynomial_sampler.x for reproductions. Verify the suggested root-cause analyses and patches across the generics, new-style proc, and IR-lowering issues; completion requires verified fixes for the relevant constituent bugs.

Written by the indexing model from the issue text.