perf(sdui): cache visible row list and eliminate rows.any linear scan in SduiTreeBuilder.build() #1359

Summary

In lib/features/connections/sdui_tree_builder.dart:

1. Uncached tree traversal on every build:
   In build() (lines 188–189):
   final rows = _flattenVisible();
   Every time SduiTreeBuilder rebuilds (e.g. on row hover, selection change, scroll, or parent rebuild), _flattenVisible() walks the entire expanded tree and allocates a new List<_VisibleRow> with individual _VisibleRow.node objects for every visible node.
2. Linear rows.any scan:
   In line 194:
   itemExtent: rows.any((r) => r.isError) ? null : _rowExtent,
   It runs rows.any(...), which traverses the entire flattened row list on every build. Meanwhile, _expandErrors is a Map<String, String> tracked in state, so checking _expandErrors.isNotEmpty is an (1)$ check that completely eliminates this list scan.

Proposed Solution

1. Cache _visibleRows in state and only recompute when tree roots, expanded set, or load errors change.
2. Replace rows.any((r) => r.isError) with _expandErrors.isNotEmpty.

Acceptance Criteria

• Hovering over tree rows or updating selection does not re-walk the tree hierarchy or re-allocate visible rows.
• itemExtent resolution uses (1)$ _expandErrors.isNotEmpty check.
• Expanding/collapsing nodes and error retries continue to function identically.