fix: correct broken ISIN checksum (validator accepted any check digit)

[DucMinhNe]

Problem

_isin_checksum never actually validates the check digit. It computes a per-character val inside the loop but never adds it to check:

check, val = 0, None
for idx in range(12):
    ...
    val = ...        # computed
    if idx & 1:
        val += val   # still never added to check
return (check % 10) == 0   # check is always 0 -> always True

So (check % 10) == 0 is always True and isin() accepts any 12-character string with valid characters, regardless of the check digit:

isin('US0378331006')  # wrong check digit -> True (should be invalid)
isin('US0004026251')  # wrong check digit -> True
isin('AAAAAAAAAAAA')  # -> True

The existing "invalid" test cases (010378331005, XCVF, 00^^^1234, A000009) are all rejected earlier by the length / character / idx > 1 guards, so the checksum branch was effectively untested and the bug stayed hidden.

Fix

Implement the actual ISO 6166 algorithm: the check digit must be numeric, expand each letter to its two-digit value (A=10 .. Z=35), then run a right-to-left Luhn sum over the expanded digit string.

I validated the new implementation against python-stdnum's authoritative isin.is_valid over 100k+ country-code-valid random inputs plus random fuzzing: zero mismatches.

Tests

• Added wrong-check-digit cases (US0378331006, US0004026251, GB0002634947) to the invalid set. These fail on master and pass with the fix.
• Replaced JP000K0VF054 in the valid set: it is not a valid ISIN (its check digit is wrong), it only passed before because the checksum did nothing. Substituted real valid ISINs (GB0002634946, DE000BAY0017).

pytest tests/test_finance.py is green (25 passed); ruff check and ruff format --check clean.