refactor(watch): single watched-progress predicate

test/media/watch_progress_test.dart

@@ -0,0 +1,27 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:plezy/media/watch_progress.dart';
+
+void main() {
+  group('isWatchedProgress', () {
+    test('false for zero or negative duration', () {
+      expect(isWatchedProgress(positionMs: 1000, durationMs: 0, threshold: 0.9), isFalse);
+      expect(isWatchedProgress(positionMs: 1000, durationMs: -1, threshold: 0.9), isFalse);
+    });
+
+    test('exact threshold counts as watched', () {
+      expect(isWatchedProgress(positionMs: 90, durationMs: 100, threshold: 0.9), isTrue);
+    });
+
+    test('just below threshold is not watched', () {
+      expect(isWatchedProgress(positionMs: 89, durationMs: 100, threshold: 0.9), isFalse);
+    });
+
+    test('position past duration is watched', () {
+      expect(isWatchedProgress(positionMs: 110, durationMs: 100, threshold: 0.9), isTrue);
+    });
+
+    test('zero position is not watched', () {
+      expect(isWatchedProgress(positionMs: 0, durationMs: 100, threshold: 0.9), isFalse);
+    });
+  });
+}