GeezerVison

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• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/tools/notes/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
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• All existing tests are passing: https://github.com/NativeScript/NativeScript/blob/master/tools/notes/DevelopmentWorkflow.md#running-unit-tests-application.
• Tests for the changes are included - https://github.com/NativeScript/NativeScript/blob/master/tools/notes/WritingUnitTests.md.

What is the current behavior?

:first-child, :last-child, :nth-child(), :root and the other structural pseudo-classes are treated as states a view reports, like :active, and no view ever reports them, so they never match. CSS frameworks rely on them heavily: Bootstrap uses :first-child/:last-child about 300 times (the ends of list groups, button and input groups, cards), Tailwind's first:, last:, odd: and even: variants compile to them, and both declare their custom properties on :root.

What is the new behavior?

• :first-child, :last-child, :only-child, :nth-child(an+b), :nth-last-child(an+b) and the -of-type forms match from the view's position among its parent's children, using the parent's getChildIndex/getChildAt/getChildrenCount as the sibling combinators do. odd, even, 3, 2n+1, -n+3 are supported; :nth-child(an+b of S) is not and drops the selector.
• A layout re-matches the siblings a change can affect. Selectors that look back are flagged like + and ~ (so the existing insertChild/removeChild invalidation covers them). Selectors that look forward (:last-child, :nth-last-child(), :last-of-type) get matching flags (hasNextSiblingSelectors, hasFollowingSiblingSelectors), and addChild/insertChild/removeChild re-match the views before the change. Scopes without such selectors do no extra work.
• :root matches the application's root view and each modal's root view (the views core gives the ns-root and ns-modal classes), with the specificity of a pseudo-class.

Tests: css-structural-pseudo.spec.ts (10: matching, formulas, combinations with :not()/:is(), dependency flags, and re-matching on append/insert/remove in a StackLayout) and :root cases in css-selector.spec.ts.

Verified with all of these core PRs merged together (#11527–#11537): apps/automated passes on the iOS 26.4 simulator (1830 OK, 0 failed) and on an Android API 36 emulator (1692 OK; the one failure is the network-dependent IMAGE-CACHE.test_ImageCache_ValidUrl timeout, which also passes on reruns).