How do I do hreflang for a multi-store Magento setup?

Three rules: (1) Every URL must reference every other locale variant of itself, including a self-reference. (2) Each variant must point back, bidirectional. (3) Always include x-default. We generate hreflang server-side per URL (not via JS), validate it on every deploy with Screaming Frog, and emit it on canonical URLs only. Common foot-guns we prevent: hreflang on non-canonical URLs, region-only tags without language, and hreflang loops between cross-linked storefronts.