[web] Do not stop pruning a detached semantics subtree at a reparented node - #195842

Fixes #194420

When a node is detached from its parent, EngineSemanticsOwner._finalizeTree walks the detached subtree with _visitDepthFirstInTraversalOrderCanSkipSubtree to remove every descendant that was not reparented elsewhere. The callback returns false for a reparented node so that its subtree is skipped (it moved along with the node).

The visitor, however, recursed into its children with _visitDepthFirstInTraversalOrder, whose contract is "stop the whole traversal when the callback returns false". So for any node two or more levels below the detachment root, a reparented child ended the walk of its parent's child list: later siblings that were not reparented were never visited, never disposed, and stayed in _semanticsTree. In debug mode the post-update consistency check then throws Bad state: Semantics node map was inconsistent after update on that update and on every update after it; in release mode the orphaned nodes (with their DOM elements and roles) leak and stay registered in the map under their old ids.

This is what happens when a GlobalKey'd widget is moved to a new parent while its old ancestors and an old later sibling are removed in the same frame: 0 -> 1 -> 2 -> [3, 4] becomes 0 -> 5 -> 6 -> 3, and #4 was left behind.

Recurse with the subtree-skipping visitor instead, so that a false return only skips that node's descendants and the walk continues with its siblings. The doc comment now spells out the difference between the two visitors.

The regression test replays the issue's tree shape; before the fix it fails with the node map still containing the orphaned #4.

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