Add Trithemius 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

The Trithemius cipher is a polyalphabetic substitution cipher invented by German abbot Johannes Trithemius. It applies progressive Caesar shifts (0, 1, 2, …) to each successive letter, leaving non-alphabetic characters in place — equivalent to a Vigenère cipher with key "ABCDEFGHIJKLMNOPQRSTUVWXYZ...".

New codec: src/codext/crypto/trithemius.py

• Supports codec names trithemius and trithemius-cipher
• Shifts only [a-z]/[A-Z] characters; position counter advances on letters only
• Passes through digits, spaces, and punctuation unchanged

import codext
codext.encode("this is a test", "trithemius")   # → "tikv mx g ambd"
codext.decode("tikv mx g ambd", "trithemius")   # → "this is a test"
codext.encode("HELLO", "trithemius")            # → "HFNOS"

__examples__ covers exact encode/decode pairs, empty input, uppercase, non-alphabetic-only strings, and enc-dec roundtrip with random data (picked up automatically by tests/test_generated).

Original prompt

---

This section details on the original issue you should resolve

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

The Trithemius cipher is a polyalphabetic encryption method invented by the German abbot Trithemius. This code is a sequence of shifts from 0 to N shift in the alphabet.

Source: dCode

Reference

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

<agent_instructions>Propose an implementation according to the guideline in docs/pages/howto.md.</agent_instructions>

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

• Fixes Add new encoding: trithemius #27

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.