forms: include optgroup children in a select's list of options

[navidemad]

Closes #3057

HTMLSelectElement collected its options from its direct children only, so an <option> inside an <optgroup> was invisible to options, length, value, selectedIndex, selectedOptions and to form submission — while querySelectorAll("option") still found every option. A grouped select could not be read, set, or submitted.

Per HTML §the-select-element, a select's list of options is its option element descendants: its option children plus the option children of its optgroup children.

The path that changed

flowchart TD
    A["sel.value = 'b'<br/>(and options / length / selectedIndex / FormData)"] --> B{"how the options are enumerated"}

    B -->|"before — childrenIterator() / TreeWalker.Children"| C["direct children only<br/>options inside an optgroup never seen"]
    C --> D["no match<br/>selectedIndex = -1, value stays ''<br/>FormData entry dropped"]

    B -->|"after — OptionIterator / select_options walk"| E["children + optgroup children,<br/>in tree order"]
    E --> F["match<br/>value 'b', selectedIndex 2<br/>FormData entry 's=b'"]

Loading

Fix

• src/browser/webapi/collections/node_live.zig — new select_options mode; selected_options moves onto the same walk. Both now use TreeWalker.FullExcludeSelf with a filter that accepts an option whose parent is the select or an optgroup child of the select, so an option buried any deeper is still excluded.
• src/browser/webapi/collections/HTMLCollection.zig — plumbing for the new mode (union arm, iterator walker type, nextTw arm).
• src/browser/webapi/element/html/Select.zig — one OptionIterator replaces five hand-rolled child walks: effectiveOption, setValue, getSelectedIndex, setSelectedIndex, getLength, plus the index lookup in add. getOptions builds the new collection mode.

Two behavioral details that come with making grouped options visible:

• effectiveOption now asks Element.isDisabled instead of Option.getDisabled, so an option inside <optgroup disabled> is skipped when falling back to the first non-disabled option (concept-option-disabled). Without this, an option in a disabled group could become the implicit selection — only reachable now that those options are in the list at all.
• select.add(option, index) inserts into the optgroup when the option at that index lives in one, per §dom-select-add (the insertion parent is before's parent).

Tests

New fixture src/browser/tests/element/html/select-optgroup.html, registered in WebApi: HTML.Select. Covers the options collection and its iterator, the value setter/getter, option.selected, selectedIndex (including -1), selectedOptions, FormData, an explicit selected attribute inside a group, the disabled-optgroup fallback, add with an index inside a group, and that an option buried below an optgroup is not in the list.

Verification:

• TEST_FILTER='WebApi: HTML.Select#select-optgroup.html' zig build test — passes.
• Toggle-off: reverting the three production files (keeping the fixture and its registration) fails the fixture at options.length (1 vs 3), value ("" vs "b") and selectedIndex — so the fixture exercises the fix.
• zig build test — 1073/1073.
• End-to-end with the issue's reproducer: exit 1 on 1.0.0-nightly.8285 (8 checks disagree with the spec), exit 0 on this branch (both selects identical, 9/9 each).