LayoutImporter.Resolve now captures the resolved base element's children
and, for a pure-container leaf (no own children + no own media) inheriting
from a base WITH content, attaches that subtree. This pulls the nested
gmInventoryUI panels' content (paperdoll/backpack/3D-items) in through the
existing BaseElement+BaseLayoutId path. ShouldMountBaseChildren is a pure,
unit-tested predicate; it's inert for media-bearing inheritors and childless
style prototypes, so vitals/chat/toolbar are unaffected (existing importer
tests still green).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
