…ement (#447)

WinUI node-mode TreeView stringifies the node and cannot host a live
UIElement in TreeViewNode.Content (verified empirically on native ARM64),
so per-node `TreeViewNodeData.ContentElement` always rendered blank.

Adds a typed, data-driven `TreeView<T>` (TemplatedTreeViewElement<T>) — the
hierarchical peer of ListView<T>: per-node `viewBuilder` (the ItemTemplate
equivalent), `childrenSelector` for hierarchy, `keySelector` for identity.
OnItemInvoked/OnExpanding hand the developer's own T back. Heterogeneous
nodes + per-shape templates fall out of a switch in the viewBuilder.

Hosting follows WinUI's own mechanism: each node's view is mounted once and
kept on an attached property; the internal TreeViewList's
ContainerContentChanging hosts it on realize and releases it (Content=null)
on recycle. The host is deferred to a low-priority dispatch and re-validates
the container still belongs to the node, so it never perturbs the
synchronous expand pass (which otherwise re-prepared the expanding node with
a stale IsExpanded and snapped it shut). Expansion is uncontrolled — the
selector is only pushed when its value changes, never clobbering user toggles.

`TreeViewNodeData.ContentElement` is marked [Obsolete] pointing to
TreeView<T>; the legacy path stays functional (warning-suppressed) for
back-compat.

Samples: DataTemplateDemo §4 migrated to TreeView<T> (heterogeneous pet
groups vs. animals); gallery TreeViewPage gains a file-explorer card
(folders/docs/images via per-shape templates) and switches source snippets
to raw-string literals so they render multi-line.

Tests: TemplatedTreeViewFixtures (render, events resolve T, keyed update,
expansion-not-clobbered, collapse/expand cycle, expand-collapsed-node,
value-type T) + WinUITreeViewNativeProbe documenting native node-mode
behavior. Full self-test suite green (1075 fixtures).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

… TreeView<T> hosting declarative

Two expand/collapse fixes:

1. Legacy TreeView (the "expand flashes open then shut" regression that
   repros on main): DiffTreeViewNodes reset each live node's IsExpanded to
   the static TreeViewNodeData value on every reconcile. When a reconcile
   fires after the user toggles a node — e.g. an OnExpanding lazy-load that
   updates state, or any unrelated state change while nodes are data-expanded
   — it snapped the node back to the data's state. Now expansion is treated as
   uncontrolled: the data value is only applied when it actually changes
   between renders, never clobbering the user's (or WinUI's native) toggle.

2. TreeView<T>: applied the same uncontrolled-expansion rule, and reverted the
   experimental ContainerContentChanging hosting back to the declarative
   {Binding Content} template. The imperative host/recycle handler perturbed
   WinUI's synchronous expand pass (manifested as expand opening then closing,
   and child-click collapsing the parent); the binding never perturbs native
   expand/collapse.

Tests: legacy + typed expansion-not-clobbered guards, collapse/expand cycle,
expand-collapsed-node, constrained-viewport expand. Full suite green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>