Product Market Availability (Territory Exclusions)

A store selling one range into many markets sometimes may not sell a handful of products in particular territories: exclusive distribution agreements, licensing, regulation. This adds a product-level market availability mode (all markets, all except, only) plus exception rows naming markets, composed with channel publications on the read path. Restricted products are fully hidden in excluded markets across listings, search, collections, feeds and direct URL, with a completion validation covering a mid-cart market change. Exceptions-only storage, zero backfill.

Plan: https://github.com/spree/spree/blob/main/docs/plans/6.1-product-market-availability.md

Tracked in Linear as V-3618.