[GeoffreyRdn]

Summary

• Add JapanMyNumberChecksum and register it as a secondary validator.
• Reuse the weighted modulo-11 helper with the algorithm defined in Article 5 of Japanese ordinance No. 85.
• Cover published and generated examples, spaced formatting, invalid checksums/lengths, and remainder-0/1 edge cases.

[datadog-prod-us1-5]

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