Wayfinder: chart the Shopify headless commerce map

The route from "mozimo.in displays Shopify products" to "mozimo.in sells them",
as a plan with a phase spine and gates, plus the open decision tickets that
still shape it and the evidence they rest on.

- .scratch/shopify-headless-commerce/: plan.md (phases and gates), map.md (the
  wayfinder map), 10 open decision tickets
- CONTEXT.md: Storefront / Theme store / Cart / Parity vocabulary
- docs/adr/0001-0003: Checkout stays Shopify-hosted, one Variant per product,
  the checkout domain stays shop.mozimo.in
- docs/research/ and docs/shopify-catalog-inventory.md: verified store facts,
  the Cart API probe, and the catalogue inventory

Raw Shopify doc scrapes under .scratch/shopify-research/ are ignored: they are
working material, not record.

.scratch/shopify-headless-commerce/issues/02-cart-persistence-and-hydration.md

@@ -0,0 +1,19 @@
+# Cart persistence and SSR hydration
+
+`Type:` grilling (HITL)
+`Status:` open
+`Blocked by:` 01
+
+Part of [the map](../map.md).
+
+## Question
+
+How is a Cart identified and carried across requests? Shopify addresses a Cart by ID, and this Storefront renders on the server, so the mechanism has to survive both.
+
+Decide: where the Cart ID lives (an httpOnly cookie the server sets, client storage, or both), what its attributes are, and how the client hydrates without a flash of an empty Cart.
+
+Decide the failure paths too, because both are reachable: a Cart expires after 30 days, and Shopify deletes it on checkout completion — so a returning buyer can arrive holding a cookie that points at nothing. What happens then — silent re-create, cleared Cart, or an error?
+
+## Answer
+
+<!-- filled on resolution -->