v0.2.0 — Compare products across chains (GTIN helpers)

Compare across chains

Barcoded products share the same GTIN barcode everywhere, so it is the natural key for lining the same SKU up across chains.

• group_by_gtin({"super-pharm": sp, "shufersal": sh}, min_sources=2) — join the same product across chains, canonicalizing UPC-12 / EAN-13 to GTIN-14 so encoding differences don't split one product in two.
• item.gtin (canonical GTIN-14, or None) and item.is_barcoded on every price/promotion row. gtin rides along in model_dump() / to_df() / JSON as the join key.
• to_gtin14() / is_valid_gtin() as standalone helpers.

item_code stays raw and faithful to the source; loose/weighted goods (fresh produce, deli — no shared barcode) resolve to None by design.

Thanks to the PyWeb-IL community for the nudge.