Add Luhn / Luhn Mod N checksum 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

Implements the Luhn Mod N algorithm — a checksum scheme used to validate credit card numbers, IMEI numbers, and similar identifiers. Encoding appends a check character; decoding verifies it and strips it.

• src/codext/checksums/luhn.py: Factory-based codec (_luhn_encode(n) / _luhn_decode(n)) parametrized via the add() pattern capture group. Doubling step uses the Mod N formula d = code*2; result = d % N + d // N. Raises LuhnEncodeError / LuhnDecodeError on invalid input/checksum in strict mode.
• src/codext/checksums/__init__.py: Wires in the new module.

Supported names: luhn, luhn-10, luhn10 (standard mod-10, digits 0-9); luhn-<N> / luhn<N> for N=2..36 using the first N characters of 0-9A-Z.

import codext

codext.encode("7992739871", "luhn")    # → '79927398713'  (classic Luhn test vector)
codext.decode("79927398713", "luhn")   # → '7992739871'

codext.encode("DEADBEEF", "luhn-16")  # → 'DEADBEEFC'
codext.decode("DEADBEEFC", "luhn-16") # → 'DEADBEEF'

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Original prompt

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This section details on the original issue you should resolve

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

The Luhn algorithm, also known as the Luhn formula, “modulus 10”, or “mod 10” algorithm, is a simple checksum formula used to validate identification numbers. The algorithm – developed by IBM engineer Hans Peter Luhn – can be used to validate credit card numbers, International Mobile Equipment Identity (IMEI) numbers, and other numerical data. Businesses frequently rely on the Luhn algorithm to increase the accuracy and reliability of their data entry processes, especially when handling sensitive information such as payment data.

Source: Stripe

Reference

https://bitcoinwiki.org/wiki/luhn-mod-n-algorithm</issue_description>

<agent_instructions>Implement this checksum algorithm 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: luhn #25

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