Add Playfair cipher codec

[Copilot]

Checklist

• No unrelated changes
• Codec is new (not already implemented)
• Tests included (if cannot be automated with tests/test_generated)
• Documentation (included in the right page in docs/pages/enc)

Description

Playfair is a classical symmetric cipher invented by Charles Wheatstone (1854), using bigram substitution on a 5×5 key grid (I/J share a cell). Encoding rules: same row → shift right, same column → shift down, rectangle → swap columns. Decoding reverses each rule. Repeated-letter pairs receive an X filler (Q if the letter is X); odd-length input gets a trailing filler.

Reference: https://www.dcode.fr/playfair-cipher

New codec: crypto/playfair

• Pattern: ^playfair(?:[-_]cipher)?(?:[-_]([a-zA-Z]+))?$
  • playfair — default alphabet, no key reordering
  • playfair-<key> — keyword-derived grid
• J→I normalization applied on both encode and decode
• Non-alpha characters are stripped (cipher operates purely on letter pairs)

import codext

# Classic Wikipedia example (key="PLAYFAIREXAMPLE")
codext.encode("HIDETHEGOLDINTHETREESTUMP", "playfair-playfairexample")
# → 'BMODZBXDNABEKUDMUIXMMOUVIF'

codext.decode("BMODZBXDNABEKUDMUIXMMOUVIF", "playfair-playfairexample")
# → 'HIDETHEGOLDINTHETREXESTUMP'  (X filler visible where EE was split)

# Roundtrip (no repeated-letter pairs, even length)
codext.decode(codext.encode("INSTRUMENT", "playfair-keyword"), "playfair-keyword")
# → 'INSTRUMENT'

__examples__ covers enc, dec, and enc-dec roundtrip; auto-registered via crypto/__init__.py.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Add new encoding: playfair</issue_title>
<issue_description>### Description

The Playfair cipher is a symmetric encryption method based on polygram substitution using grids. The Playfair cipher is a symmetric encryption method using polygram substitution using bigrams (pairs of letters), invented in 1854 by Charles Wheatstone, but popularized by his friend Lord Playfair, hence its name.

Reference

https://www.dcode.fr/playfair-cipher</issue_description>

<agent_instructions>Refer to the guideline at docs/pages/howto.md.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Add new encoding: playfair #37

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