perf(sdui): virtualize SduiTreeBuilder (no full Column of expanded nodes)

[ZhuchkaTriplesix]

Parent

Part of #414

Problem

SduiTreeBuilder builds a SingleChildScrollView + recursive Column of all expanded nodes. _replaceNode rebuilds ancestor lists on expand. Large schemas → O(visible tree) widgets and O(tree) copies per expand.

Evidence

• lib/core/sdui/sdui_tree_builder.dart (~91–125, expand path)

Acceptance

• Only viewport-visible rows are built (ListView / flattened visible list)
• Expand/collapse does not deep-copy entire tree unnecessarily
• Smoke with large mock tree (thousands of nodes)

Suggested fix

Flatten visible rows; ListView.builder; mutate children in place or index by id.

[ZhuchkaTriplesix]

Shipped on dev via #435 (Closes did not auto-close because PRs targeted dev).