perf(workspace): resolve UI freezes, unmemoized allocations, and isolate blocking in grid and inspector #702

Description

Following a performance and UI freeze audit of the workspace data grid, staging buffer, and value inspector, several bottlenecks causing UI freezes, main isolate blocking, and frame drops (jank) have been identified.

Identified Bottlenecks & Proposed Solutions

1. P0: Memory allocations & broken memoization in DataGridStagingBuffer.effectiveRows

• Files: lib/features/workspace/data_grid_staging_buffer.dart, lib/features/workspace/results_tab.dart, lib/features/workspace/result_grid_view.dart
• Problem: effectiveRows clones the entire dataset (List<String>.from) on every getter access. When ResultsTab.build() passes effectiveRows to _getFilteredRows, the reference check identical(_memoEffectiveRows, effectiveRows) fails on every single rebuild (hover, cursor blink, focus change). This triggers full synchronous re-filtering (GridFilterEngine.filterRowIndices) on 50,000+ rows repeatedly. In addition, column auto-fit access triggers repeated list allocations.
• Fix:
  • In DataGridStagingBuffer, return baseRows directly when !isDirty (zero allocation, stable reference).
  • Cache _cachedEffectiveRows when dirty, and invalidate only upon actual mutations (updateCell, deleteRow, addRow, revertAll).

2. P1: Synchronous $O(N \log N)$ sorting on the main isolate

• Files: lib/features/workspace/result_grid_view.dart
• Problem: sortResultGridRowsWithIndices performs key parsing and multi-column sorting synchronously on the UI thread. On tables with 50,000–100,000+ rows, sorting freezes the entire desktop UI for 500ms–2s+.
• Fix:
  • For small row counts (< 3000 rows), maintain fast synchronous sorting.
  • For large row counts (>= 3000 rows), offload sorting to compute / Isolate.run to keep UI fluid and responsive.

3. P1: GridGroupingsEngine.buildGroups executed directly in build()

• Files: lib/features/workspace/data_grid_groupings_view.dart
• Problem: Group tree generation, recursive map hashing, and aggregation calculations run directly inside build(). Any parent rebuild or layout resize triggers expensive multi-pass iteration over all rows.
• Fix: Cache the grouping tree (_cachedGroups) in the state and only recompute when rows, group columns, or aggregates change (didUpdateWidget).

4. P2: Synchronous jsonDecode / XML validation on every keystroke

• Files: lib/features/workspace/data_grid_value_panel.dart
• Problem: _validateContent executes jsonDecode on every keystroke without debouncing. Editing large JSON/XML cells (100KB+) freezes typing and causes visual lag.
• Fix: Add a 150ms debounce timer for validation parsing while typing.

Acceptance Criteria

• DataGridStagingBuffer.effectiveRows avoids allocations when clean and caches rows when dirty.
• ResultsTab._getFilteredRows memoization correctly preserves cache across frame rebuilds.
• Large table sorting (>= 3000 rows) does not block the UI isolate.
• DataGridGroupingsView caches grouping tree results outside of build().
• DataGridValuePanel debounces validation parsing during text entry.
• All workspace tests and analysis pass without regressions.