Select::options and optgroup bugfix

[g105b]

I found a real inconsistency: $select->options already walks descendant <option> elements, but $select->elements still returns only direct children, and length is derived from elements.

That means <optgroup> breaks the select’s option model in part of the API. I'm fixing that at the shared collection layer instead of patching one property in isolation.

The bug is fixed in HTMLElement. <select> now builds a single shared HTMLOptionsCollection from descendant option elements in tree order, so options, elements, and length stay consistent when options are nested inside <optgroup>.

I also tightened the regression test in HTMLSelectElementTest so it assetrs both $select->options ordering and $select->length across two optgroups.