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

Implements the Vigenère cipher — a polyalphabetic substitution cipher that applies a series of interwoven Caesar shifts driven by a repeating keyword. Non-alpha characters pass through unchanged; case is preserved per character.

New file: src/codext/crypto/vigenere.py

• Pattern: vigen[eè]re[-_](?:cipher[-_])?KEY — accepts both - and _ separators, the accented spelling (vigenère), and an optional cipher infix
• Key is normalised to lowercase; non-alphabetic or empty keys raise LookupError
• __examples__ covers the canonical Wikipedia test vector, case preservation, and round-trip fidelity

import codext

codext.encode("ATTACKATDAWN", "vigenere-lemon")  # → "LXFOPVEFRNHR"
codext.decode("LXFOPVEFRNHR", "vigenere-lemon")  # → "ATTACKATDAWN"

codext.encode("Hello World", "vigenere-key")     # → "Rijvs Uyvjn"

• Fixes Add new encoding: vigenere #23

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