perf(grid): lazy overlay list in DataGridStagingBuffer to eliminate 50,000-row allocations on cell edits #1538

Problem

In DataGridStagingBuffer.effectiveRows (lib/features/workspace/data_grid_staging_buffer.dart), whenever a user edits a single cell or inserts a row, _buildRows is invoked.
This creates a full copy of the entire table (result = <List<String>>[]) copying 50,000 pointers, wrapped in List.unmodifiable.
During fast keyboard typing in an editable grid cell, this continuously allocates 50,000-element lists in the heap, causing garbage collection spikes.

Solution

• Implement a lazy overlay row list (_LazyOverlayRowList implementing ListBase<List<String>>).
• Return _originalRows[index] for unedited rows and the modified row overlay for edited rows directly in (1)$.
• Eliminate the allocation of the full row list on every cell keystroke.

Benefits

• 0 full list allocations when editing grid cells.
• Instant, zero-latency cell mutations regardless of table size (10k-100k rows).