[Toolkit][Shadcn] Align select with shadcn reference

[laurentdav]

Q	A
Bug fix?	no
New feature?	yes
Deprecations?	no
Documentation?	yes
Issues	Part of #3233
License	MIT

Depends on #3743.

The existing Shadcn select recipe renders a native HTML control, while the upstream Select is a custom composite component triggered by a button. This aligns the recipe with the current upstream component and leaves native browser behavior to the new native-select recipe.

The updated recipe adds the composed Select:Trigger, Select:Value, Select:Content, Select:Group, Select:Label, Select:Item, and Select:Separator API. It supports form submission, required validation, disabled states, keyboard navigation, typeahead, item-aligned and popper positioning, scroll controls, and RTL layouts. Trigger composition follows the Toolkit asChild attributes convention.

All current upstream examples are included in the recipe README and covered by regenerated snapshots. Existing native-select usages in the Field, Input, and Pagination examples now use NativeSelect.

This draft is stacked on #3743 so its CI can render those migrated examples. Once #3743 is merged, this PR will contain only the select recipe alignment and its dependent example updates.

No companion site change is required: the current site auto-registers Toolkit controllers and scans the installed Shadcn kit during its Tailwind build.

Tests:

• bin/ux-toolkit-kit-lint --fail-on-warning kits/shadcn
• pnpm exec oxfmt --check on the changed sources
• pnpm exec oxlint src/Toolkit/kits/shadcn/select/assets/controllers/select_controller.js
• php -d memory_limit=-1 vendor/bin/phpunit (1089 tests, 2634 assertions)