Add Beaufort 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

Adds the Beaufort cipher — a polyalphabetic substitution cipher using C = (K - P) mod 26 (key minus plaintext, vs. Vigenère's plaintext plus key). Self-reciprocal: encode and decode are the same operation.

• src/codext/crypto/beaufort.py: New codec modelled after vigenere.py. Supports key via beaufort-<key> / beaufort_<key> naming. Since the cipher is self-reciprocal, beaufort_decode = beaufort_encode.
• src/codext/crypto/__init__.py: Registered beaufort alongside the other crypto codecs.

import codext
codext.encode("ATTACKATDAWN", "beaufort-lemon")  # → "LLTOLBETLNPR"
codext.decode("LLTOLBETLNPR", "beaufort-lemon")  # → "ATTACKATDAWN"

Original prompt

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This section details on the original issue you should resolve

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

The Beaufort cipher is a polyalphabetic encryption system very similar to the Vigenère cipher, but based on a different operation: instead of adding the key to the plaintext, the plaintext is subtracted from the key.

Reference

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

<agent_instructions>Please propose an implementation according to the guideline at docs/pages/howto.md.</agent_instructions>

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

• Fixes Add new encoding: beaufort #29

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