feat(dc): Add multi-metric transformation feature (direction 1+2)

Summary:
- Implement intelligent multi-metric grouping detection algorithm
- Add direction 1: timepoint-as-row, metric-as-column (analysis format)
- Add direction 2: timepoint-as-column, metric-as-row (display format)
- Fix column name pattern detection (FMA___ issue)
- Maintain original Record ID order in output
- Add full-select/clear buttons in UI
- Integrate into TransformDialog with Radio selection
- Update 3 documentation files

Technical Details:
- Python: detect_metric_groups(), apply_multi_metric_to_long(), apply_multi_metric_to_matrix()
- Backend: 3 new methods in QuickActionService
- Frontend: MultiMetricPanel.tsx (531 lines)
- Total: ~1460 lines of new code

Status: Fully tested and verified, ready for production

frontend-v2/src/modules/dc/pages/tool-c/components/Toolbar.tsx

@@ -131,7 +131,7 @@ const Toolbar: React.FC<ToolbarProps> = ({
       {/* 高级按钮（Phase 3） */}
       <ToolbarButton
         icon={ArrowLeftRight}
-        label="长→宽表"
+        label="长↔宽表"
         colorClass="text-indigo-600 bg-indigo-50 hover:bg-indigo-100"
         onClick={onPivotClick}
         disabled={!sessionId}