polish(ux): selected filter chips fill solid for clear contrast

The selected state used a pale tonal container (seedPrimaryContainer)
that sat almost on top of the pale canvas, so 'on' barely read. Selected
chips now fill solid — seedGreen for the plain attribute/market chips and
the swatch's own ink for family/form chips — with white label, icon and
checkmark. Idle chips stay light with dark ink, so the on/off jump is
unmistakable. White-on-ink AA is locked by a new contrast test.

apps/app_seeds/test/ui/category_palette_contrast_test.dart

@@ -35,4 +35,19 @@ void main() {
           reason: '$type ink on fill');
     }
   });
+
+  // A *selected* swatch chip fills solid with its ink and draws white content
+  // (label, icon, checkmark). That pairing must clear AA too.
+  test('white content meets AA on every selected (ink-filled) swatch', () {
+    for (var i = 0; i < 60; i++) {
+      final s = categorySwatch('family-$i');
+      expect(contrast(Colors.white, s.ink), greaterThanOrEqualTo(4.5),
+          reason: 'white on category-$i ink');
+    }
+    for (final type in LotType.values) {
+      final s = lotTypeSwatch(type);
+      expect(contrast(Colors.white, s.ink), greaterThanOrEqualTo(4.5),
+          reason: 'white on $type ink');
+    }
+  });
 }