fix(typography): bake the prototype colour into eyebrowLabel

The prototype's eyebrows are always rgba(242,247,250,.42) (t4 lines
254, 299, 381, 450, 511, 660). eyebrowLabel set size/weight/letter-
spacing only, no colour, so every call site (settings group headers,
vacation section titles, the ringing screen's snooze label) rendered
at whatever full-opacity default the ambient text theme resolved to.

One factory-level change fixes every current and future consumer of
PluriWaveTypography.eyebrowLabel — no call site needs editing.

S9, Tier 1 visual-fidelity pass (audit id 2521).

test/tema/pluriwave_typography_test.dart

@@ -65,6 +65,15 @@ void main() {
       expect(type.eyebrowLabel.letterSpacing, 0.8);
     });
 
+    test('S9 (Tier 1 visual fidelity): eyebrowLabel bakes in the prototype '
+        "colour rgba(242,247,250,.42) — it used to carry no colour at all, "
+        'so it rendered at full onSurface opacity', () {
+      expect(
+        type.eyebrowLabel.color,
+        const Color(0xFFF2F7FA).withValues(alpha: 0.42),
+      );
+    });
+
     testWidgets('context.pluriType exposes the registered extension', (
       tester,
     ) async {