Fix a bug in cohomology calculations

[stertooy]

Closes #211. Built on top of #210.

The old condition was meant to check if "conjugation by the inverse of a generator" works correctly, by confirming it's the inverse operation by the (already verified) "conjugation by a generator". It checked this by testing $(x^y)^{y^{-1}} = x$ for generators $x,y$.

However, conjugating $x^y$ by $y^{-1}$ means we interpret $x^y$ as a word, and apply "conjugation by $y^{-1}$" to each of the generators appearing in this word. This means it might use the same relation multiple times, and an error in the tail could then appear an even number of times, so mod 2 it would disappear and pass the check.

Now, we instead check that $(x^{y^{-1}})^y = x$ (or rather $(x^{y^{-1}})y = yx$). Since we apply "conjugation by an inverse" to a single generator instead of a word, we can't have multiple errors cancelling out each other.

AI Disclosure: bug was discovered by GPT-6 Astra during a package audit.

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 67.75%. Comparing base (89138f3) to head (36f17bc).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
gap/cohom/twocohom.gi	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #169      +/-   ##
==========================================
+ Coverage   67.72%   67.75%   +0.02%     
==========================================
  Files          88       88              
  Lines       12914    12913       -1     
==========================================
+ Hits         8746     8749       +3     
+ Misses       4168     4164       -4     

Files with missing lines	Coverage Δ
gap/cohom/twocohom.gi	74.32% <83.33%> (-0.09%)	⬇️

... and 2 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[fingolfin]

I took the liberty of rebasing this on latest master, so that the diff became small.