namedLayoutSlots: named slots are silently dropped when a page renders without a layout #30

Split out of #29.

LayoutSlotsForward renders only ctx.slots.default, so when there is no registry above the page, everything it declares for the layout disappears — no error, no warning, the markup is just not in the tree.

This mostly hurts unit tests: mountSuspended(Page) renders a page outside NuxtLayout, so a page test silently sees less than production. Today the only way around that is to build a registry host by hand in every spec.

Inlining the slots in the page was the wrong fix, as you said in #29. Maybe this belongs in @nuxt/test-utils instead — something that kicks in only when namedLayoutSlots is enabled and mounts a page the way the app renders it.