fix(plot-builder): auto-smart groups by polar condition, not per-alpha

Alpha is the sweep variable for polars—it should never be a grouping
key in Auto (Smart) mode.  Removes autoGroupInvariant from alpha and
excludes it from label generation so CD-vs-CL plots group by
(airfoil, Re, Mach, …) instead of exploding into one series per point.

Made-with: Cursor

Author: aeronauty

flexfoil-ui/src/lib/plotFields.ts

@@ -13,7 +13,7 @@ export const PLOT_FIELDS: PlotFieldMeta[] = [
   { key: 'id', label: 'ID', kind: 'numeric' },
   { key: 'airfoil_name', label: 'Airfoil', kind: 'categorical', autoGroupInvariant: true },
   { key: 'airfoil_hash', label: 'Airfoil Hash', kind: 'categorical', autoGroupInvariant: true },
-  { key: 'alpha', label: 'Alpha', kind: 'numeric', autoGroupInvariant: true },
+  { key: 'alpha', label: 'Alpha', kind: 'numeric' },
   { key: 'reynolds', label: 'Re', kind: 'numeric', autoGroupInvariant: true },
   { key: 'mach', label: 'Mach', kind: 'numeric', autoGroupInvariant: true },
   { key: 'ncrit', label: 'Ncrit', kind: 'numeric', autoGroupInvariant: true },

flexfoil-ui/src/lib/polarDetection.ts

@@ -257,7 +257,7 @@ export function detectSmartRunGroups(
 
   const labels = buildSmartLabels(
     allSegments.map(s => s.meta),
-    new Set(plottedFields as string[]),
+    new Set([...(plottedFields as string[]), 'alpha']),
   );
 
   return allSegments.map((seg, i) => {