Preview behavioral QA: accessibility and variant-to-checkout checks

Problem: Nothing exercises a Spark theme's commerce behavior before handoff. The variant-to-cart-to-checkout-handoff path and accessibility are checked by hand, so a visually plausible theme that loses those contracts is only caught by a person, if at all.

Evidence: no browser-level checks in the repo; the next-theme-dev DOM smoke snippet explicitly does not replace add-to-cart QA; accessibility passes in PLAN.md:64-73 (W3) remain to-be-done.

Done when:

• A scripted check runs against an unpublished-theme preview URL: select a variant, add to cart, open the cart, verify the checkout handoff link, and run an axe accessibility pass on key templates.
• The script produces a recorded pass/fail verdict artifact per run and is documented as the pre-handoff QA step.
• A first baseline run over the stock theme is recorded, with findings filed.

Depends on: a dev store and theme credentials provisioned by a human; the greenfield bootstrap issue in the skills repo for the preview URL derivation.

Review PLAN.md:64-73 and the existing next-theme-dev DOM smoke snippet first. Define the browser-level run against an unpublished-theme preview URL, covering variant selection, add-to-cart, cart, checkout handoff, and axe checks on key templates. Done means a recorded pass/fail artifact, documentation of the pre-handoff step, and a baseline stock-theme run with findings filed.