Issue #3611985: Add a yoyaku_cart module: a cross-surface basket and checkout

A cross-surface booking cart. yoyaku_cart (core-only): BasketSummary (events -> priced tickets), CartForm at /booking/cart (event cards, per-ticket Remove, Checkout), CartCheckoutEvent (the seam), and hook_yoyaku_cart_line_alter() (a line's owning module names its ticket; yoyaku_seating labels seats, no dependency either way). yoyaku_cart_orchestra submodule: a CartCheckoutEvent subscriber that starts the order workflow and redirects to its first step. Also BookingCart::releaseLine() for per-line removal, and the yoyaku_seating seat-label contribution. Kernel-tested (summary shape, describe hook, releaseLine). Surface wiring (links from slot/seating pages, dropping the per-slot checkout button) is a follow-up.