Light-DOM slot contract

There is no shadow DOM and therefore no native <slot>. Instead:

• Elements capture their author-supplied children once, when first connected (captureLightDom()).
• Tier-A elements (layout, card, badge…) only add classes to the host; children stay where they are.
• Tier-B elements (select, tabs, modal…) build internal DOM and move the captured children into it. Named slots use the slot="…" attribute (for example slot="footer" on ui-modal).
• Changing children after first connect is not observed. Use properties (options, setItems) instead.

Never wrap the direct children of ui-shell: its layout is a CSS grid over those children.