fix(focus): skip destroyed elements in restoreFocus

[chiefcll]

Problem

restoreFocus from useFocusStack would give focus to an element saved by storeFocus even after that element had been destroyed (e.g. the page it lived on was removed).

Fix

• restoreFocus now pops destroyed elements off the stack and focuses the most recent element that still exists. If every stored element is destroyed, it returns false and leaves focus unchanged.
• DOM renderer: element.destroyed reads the flag from the renderer node. The WebGL CoreNode sets it on a node and its whole subtree, but DOMNode never set it, so the check could never fire in DOM rendering mode (or in the jsdom tests). DOMNode now sets destroyed on itself and its subtree when destroyed, matching CoreNode.

Side effect

KeepAlive already checks .destroyed before reusing a cached child. In DOM rendering mode that check never fired before; it now does, so destroyed children get rebuilt instead of reused, the same as with WebGL.

Known gap (unchanged)

A removed element is only destroyed in the post-mutation flush. If restoreFocus runs in the same synchronous step that removes the element (e.g. inside that page's onCleanup), it isn't marked destroyed yet and still receives focus.

Tests

tests/focusStack.test.tsx:

• A stored page and a stored child inside it are removed → restoreFocus skips both, focuses the earlier element, and the skipped entries are gone from the stack.
• Every stored element destroyed → restoreFocus returns false and focus stays put.

Both tests fail on main and pass with this change. Full suite, tsc and lint pass.

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