Tryst::UI::RealizedNode

What a Node gets in its realized slot once the realizer creates a live widget for it: which app owns it, plus three Tk paths that are the same string for an ordinary widget and only diverge for the two scrolling cases. Keeping them separate is what lets the rest of the realizer stay oblivious to which case it's looking at.

• path is the widget itself - what a Handle acts on (#configure, event bindings, on_close, ...).
• arrange_path is what the parent's own layout places. It differs for a node auto-wrapped in a scrollbar (a bare list/text_area/ table/tree/canvas, when scrolling applies - see Realizer#create_native_scrollable): path stays the real widget so a Handle keeps acting on it directly, but the widget's actual Tk parent is now the wrapper frame holding the scrollbar, and that wrapper is what has to be packed/gridded into the surrounding layout.
• content_path is where this node's CHILDREN belong. It differs for a :scrollable (Realizer#create_scrollable), whose children live in an embedded canvas viewport rather than under its own path - which is a plain frame whose slots the scrollbar's grid already owns. Anything creating a child into an already-realized parent (Realizer#realize_subtree, driving Session#add) has to build into this rather than path, or the child lands in the wrong parent under the wrong geometry manager.

content_bindtag is the odd one out - not a path, but the same idea: a Tk bindtag every descendant of this node is expected to carry, so a subtree added after realize can be brought up to the same footing as one created during it. Only a scrolling :scrollable sets it (see Realizer#wire_wheel_scroll for what the tag is for); nil everywhere else, including a :scrollable with both axes turned off.

app is AppContract (see app_contract.cr), not the concrete Tryst::App, so a Realizer built against FakeApp for a headless spec produces RealizedNodes usable exactly the same way as a real one - and nilable, matching how ruby-tryst's own tests construct this (RealizedNode.new(app: nil, path: ...)) for App-free Document/Node coverage.