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.

docs/adr/0001-checkout-stays-shopify-hosted.md

@@ -0,0 +1,10 @@
+# Checkout stays Shopify-hosted
+
+Checkout for the Storefront is Shopify's hosted checkout, reached by following a Cart's `checkoutUrl`. We are not building a custom checkout.
+
+Shopify sunset the Checkout APIs in 2025-04 — no `checkout*` mutation survives in the Storefront API — so a merchant-built checkout is no longer a supported path. Checkout UI extensions and checkout styling are Plus-only, and this store is on **Grow**. The one documented exception, the Shop Pay Wallet / Shop Components API, is Shop-Pay-only and states that it cannot be used on an existing store.
+
+## Consequences
+
+- The buyer's origin changes from `mozimo.in` to `shop.mozimo.in` at payment. That is a brand-owned subdomain, not a `*.myshopify.com` host.
+- Anything requiring Checkout to be rendered, extended or styled by us is out of reach until the store moves to Plus — at which point it becomes a fresh effort, not a resumption of this one.