fix(ar/cbu): accept valid CBUs whose check digit is 0

[nicolasfishman]

Problem

AR.cbu rejects valid CBUs whenever either of the two check digits is 0.

const { stdnum } = require('stdnum');

stdnum.AR.cbu.validate('0720429088000002339140');
// => { isValid: false, error: InvalidChecksum }   ❌ this CBU is valid

stdnum.AR.cbu.validate('0000000000000000000000');
// => { isValid: false, error: InvalidChecksum }   ❌ both check digits are 0

Root cause

In src/ar/cbu.ts the check digits are computed as:

const s1 = String(
  10 -
    weightedSum(front, { reverse: true, weights: [3, 1, 7, 9, 3, 1, 7], modulus: 10 }),
);

weightedSum(..., { modulus: 10 }) already returns a value in [0, 9], so 10 - that
is in [1, 10]. When weightedSum returns 0, 10 - 0 === 10 and String(10) is
"10", which can never equal the single-character check digit c1/c2 ("0"). Any CBU
whose bank block or account block has a check digit of 0 is therefore rejected.

Worked example for 0720429088000002339140:

• front 0720429, c1 = "0", weightedSum(front) = 0 → s1 = "10" (should be "0")
• back 8800000233914, c2 = "0", weightedSum(back) = 0 → s2 = "10" (should be "0")

Fix

Wrap the result modulo 10 — the same thing the sibling src/mx/clabe.ts validator
already does ((10 - weightedSum(...)) % 10), so this just brings ar/cbu in line with it.

Tests

Added three cases to src/ar/cbu.spec.ts:

• 0720429088000002339140 — both check digits 0 → now valid
• 0000000000000000000000 — both check digits 0 → now valid
• 0720429088000002339141 — tampered → still InvalidChecksum

Full suite passes (npx jest), lint and prettier are clean.

[gemini-code-assist]

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