perf(workspace): memoize DataGrid filtering in ResultsTab to prevent unneeded re-filtering on build #650

Motivation & Problem

In lib/features/workspace/results_tab.dart (lines 125–134), GridFilterEngine.filterRowIndices is called directly inside build():

final filteredIndices = GridFilterEngine.filterRowIndices(
  filterText: _filterText,
  columns: widget.columns,
  rows: effectiveRows,
);
final filteredRows = filteredIndices.length == effectiveRows.length
    ? effectiveRows
    : filteredIndices.map((i) => effectiveRows[i]).toList();

When query results contain 10,000 to 100,000 rows, any unrelated UI rebuild (such as hovering over buttons, tab switches, cursor blinking, or status updates) causes the UI thread to synchronously re-filter all rows and allocate a new 50,000-element list in memory, causing frame drops (jank) and unnecessary garbage collection pressure.

Proposed Solution

1. In _ResultsTabState:
   • Cache _cachedFilteredIndices and _cachedFilteredRows.
   • Track _lastFilterText, _lastEffectiveRows, and _lastColumns.
   • Recompute filtering only when _filterText, rows, or columns actually change.
2. Return cached results instantly on idle / unrelated widget rebuilds (0ms).
3. Add unit and widget tests verifying correct memoization and invalidation when filter or rows change.

Acceptance Criteria

• Unrelated widget rebuilds do not re-run GridFilterEngine.filterRowIndices.
• Changing _filterText or dataset immediately updates the cached filtered rows.
• All existing results_tab_test.dart and grid integration tests pass.