Add appendix discussing recursive variants

[jbcoe]

No description provided.

[zhihaoy]

Yea, but none of these matters. Deleting it, putting std::indirect with operator T& in the type traits in this file, and everything works as before, because neither type is exposed in the interface. It's the saved effort of modeling operations via meta-programming matters.

[codecov]

Codecov Report

❌ Patch coverage is 73.07692% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.71%. Comparing base (cea96af) to head (bae0c36).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
exploration/recursive_variant_test.cc	76.00%	6 Missing ⚠️
exploration/recursive_variant.h	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #611      +/-   ##
==========================================
- Coverage   99.51%   98.71%   -0.81%     
==========================================
  Files          12       14       +2     
  Lines         829      855      +26     
  Branches       90       90              
==========================================
+ Hits          825      844      +19     
- Misses          4       11       +7     

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[nbx8]

Appendix is a nice addition, thanks.

[Copilot]

@jbcoe I've opened a new pull request, #612, to work on those changes. Once the pull request is ready, I'll request review from you.