[FEATURE] Add Four-Square Cipher Module (Classical Digraph Cipher)

[Aakif-Kohari]

Summary

The Classical category currently has Playfair as its only digraph substitution cipher. Four-Square (Delastelle) is a related but distinct construction (uses 4 separate 5x5 grids instead of 1) that pairs naturally with the existing Playfair implementation and reuses much of its grid-construction logic pattern.

What's missing

• No lib/cipher/four-square.ts
• No visualizer entry, worker routing, or tests

Proposed implementation

• Build two keyed 5x5 grids (top-right and bottom-left) plus two standard A-Z (I/J merged) grids (top-left plaintext grid, bottom-right ciphertext grid), following the same bigram-preparation rules as Playfair (pad odd length, uppercase, strip non-alpha)
• Digraph substitution: for plaintext pair (a,b), find a in top-left grid, b in bottom-right grid, take the row/column intersection rule across the 4-grid layout
• Steps: grid construction (4 matrices) + per-bigram substitution, reusing the matrix/highlight step schema Playfair already uses
• Security status: legacy
• Test vectors: use the standard published Four-Square worked example

Acceptance criteria

• Round-trip encrypt/decrypt correct
• Matches published Four-Square test vector
• Handles odd-length input and duplicate-letter bigrams like Playfair does
• Unit + fuzz tests
• Dark/light mode support via existing theme system

Assignment

I'd like to work on this issue under ECSoC26. Will open a PR only after approval/assignment by @csxark or another maintainer.

[github-actions]

🎉 Issue Assigned

Hi @Aakif-Kohari, I'm assigning this initiative to you.

Looking forward to your contributions!

Please do the following before coding:

• ✅ Sync your fork with the latest main
• ✅ Create a new feature branch
• ✅ Implement the requested changes
• ✅ Test your work
• ✅ Open a PR referencing this issue

Wishing you the very best, and happy contributing! 🚀🚀