Fix bugs in low-index-subgroup functions (trivial cases)

[stertooy]

Part of fixing #201, extracted from #202.

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.71%. Comparing base (9a15215) to head (48295ba).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #209      +/-   ##
==========================================
+ Coverage   61.59%   61.71%   +0.12%     
==========================================
  Files          87       87              
  Lines       13010    13020      +10     
==========================================
+ Hits         8013     8035      +22     
+ Misses       4997     4985      -12     

Files with missing lines	Coverage Δ
gap/pcpgrp/findex.gi	12.77% <100.00%> (+7.63%)	⬆️
gap/pcpgrp/nindex.gi	11.96% <100.00%> (+6.60%)	⬆️

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