perf(export): stream DataExportService to disk (stop giant String)

[ZhuchkaTriplesix]

Parent

Part of #414

Problem

Primary export UI path (DataExportService.formatAsync / saveToFile) builds one giant String (CSV/JSON/Markdown/SQL) on an isolate, then File.writeAsString.

Streaming helper writeResultGridCsv already exists but is not used by this path. Wide × 10⁵-row grids → O(cells) heap + full matrix copy into the isolate.

Evidence

• lib/shared/services/data_export_service.dart (~147–219)
• Callers: results_tab.dart, extension_table_view.dart
• Unused stream: lib/core/csv/result_grid_csv.dart (writeResultGridCsv)

Related

#277 fixed sync UI-isolate CSV freezes; this is the remaining full-document materialization issue across formats.

Acceptance

• File export streams to IOSink for CSV (and preferably JSON/SQL)
• UI wires large exports through streaming path; clipboard may stay string-based with size guard
• No full-string build for file export of ≥N rows (document threshold)
• Smoke test / unit test for streamed write

Suggested fix

Reuse writeResultGridCsv; add streaming JSON/SQL writers; “export to file only” for oversized sets.

[ZhuchkaTriplesix]

Shipped on dev via #431 (Closes did not auto-close because PRs targeted dev).