Override template for a single CngxTreeSelect row. Replaces the
built-in twisty/indicator/label layout while keeping the panel's
role="treeitem" wrapper and ARIA wiring intact.
Context: CngxTreeSelectNodeContext. Carries every reactive
flag (expanded, selected, indeterminate, hasChildren, depth,
disabled) plus closed toggleExpand / handleSelect callbacks so
custom markup participates in commit / cascade / announce.