perf(results): charts and groupings parse and aggregate all rows on the UI thread on every rebuild #1170

Problem

Results can hold up to 100 000 rows (sql_result_max_rows, default 5 000). Charts and Groupings process them synchronously on the UI thread:

• ChartData.numericColumns parses every cell of every column with double.tryParse when Charts opens and on every new result (quick_chart_view.dart: _recompute).
• build() calls ChartData.build (parse all rows again) on every rebuild, no memo; SVG export parses once more.
• DataGridGroupingsView calls GridGroupingsEngine.buildGroups over all rows synchronously on every option click (column, aggregate, sort).

Scope

• Memoize chart points by (rows identity, columns, type, X, Y, aggregation).
• Above a threshold (e.g. 20 000 cells), numeric detection, chart data and groupings run in compute / Isolate.run, with QueryaSpinner in the pane; stale results discarded (generation counter), like the selection stats in perf(workspace): offload massive grid selection statistics calculation to background compute #652.
• For the All option of fix(charts): bar/line silently use the first 50 rows; add aggregation and Top N #1158: downsample line series (LTTB) to the pixel width, cap bars.

Acceptance

• Switching to Charts / changing a grouping option on 100 000 rows keeps UI frames within budget (bench scenario).
• Unit tests for the memo key and for LTTB.