Add support for European day-first date format (e.g. "23/2/26" = 23 Feb 2026)
in the sell-out CSV pipeline so months and years are correctly extracted from
column C of Amazon Sell Out 2023-2025.csv.
- normalizeMonth: detect DD/MM/YY when first part > 12, return "Mon-YY"
- mapRowToRecord: add 'C', 'Date', 'Fecha', 'DATA' to month column aliases
- validateSellOutHeaders: accept date columns as substitute for YEAR + MONTH
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>