refactor: make consistent names for row and col indexes

[walldiss]

This PR standardizes index naming for rows and columns across the codebase to rowIdx and colIdx.

Resolves #314

[codecov]

Codecov Report

Attention: Patch coverage is 86.13861% with 14 lines in your changes are missing coverage. Please review.

Project coverage is 82.96%. Comparing base (d6c118c) to head (c116d1f).
Report is 19 commits behind head on main.

Files	Patch %	Lines
extendeddatacrossword.go	75.00%	7 Missing and 2 partials ⚠️
datasquare.go	89.13%	2 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #317      +/-   ##
==========================================
+ Coverage   80.89%   82.96%   +2.06%     
==========================================
  Files           8        7       -1     
  Lines         869      628     -241     
==========================================
- Hits          703      521     -182     
+ Misses        119       65      -54     
+ Partials       47       42       -5     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.