An element forwards parts with `exportparts="a, b"` (no renaming). Inside its shadow tree two DIFFERENT inner elements both carry `part="a"`. A consumer writes `host::part(a)`. How many elements does the consumer's `::part(a)` rule match, and what is the underlying model for part name collisions?
<!-- host shadow tree -->
<div part="a"></div>
<span part="a"></span>
<!-- consumer: host::part(a){outline:1px solid} -->