Stage 2: Library search, filters and Exercise detail with Flip Animation

- LibraryFilter: word search on names; muscle (primary), equipment and
  level choices that widen within a filter and narrow across filters
- ExerciseBrowser (reusable for the Plan picker) with search field,
  filter sheets showing live result counts, a clear button and an
  empty state
- Exercise detail: start/end Flip Animation at each Mode's pace (still
  with reduced motion), facts, main/secondary muscles, numbered steps

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/exercises/library_filter_test.dart

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+import 'package:flutter_test/flutter_test.dart';
+import 'package:zenxercise/src/exercises/exercise.dart';
+import 'package:zenxercise/src/exercises/library_filter.dart';
+import 'package:zenxercise/src/modes/mode.dart';
+
+Exercise _exercise(
+  String name, {
+  List<String> primary = const ['hamstrings'],
+  List<String> secondary = const [],
+  String? equipment = 'body only',
+  String level = 'beginner',
+}) => Exercise(
+  id: name.replaceAll(' ', '_'),
+  name: name,
+  mode: Mode.workout,
+  category: 'strength',
+  level: level,
+  equipment: equipment,
+  primaryMuscles: primary,
+  secondaryMuscles: secondary,
+  instructions: const [],
+  imageCount: 0,
+);
+
+void main() {
+  final squat = _exercise('Barbell Full Squat', primary: ['quadriceps'], secondary: ['glutes'], equipment: 'barbell');
+  final curl = _exercise('Dumbbell Bicep Curl', primary: ['biceps'], equipment: 'dumbbell', level: 'intermediate');
+  final deadlift = _exercise('Romanian Deadlift', primary: ['hamstrings'], equipment: 'barbell', level: 'expert');
+  final mystery = _exercise('Mystery Move', primary: ['glutes'], equipment: null);
+  final all = [squat, curl, deadlift, mystery];
+
+  List<Exercise> apply(LibraryFilter f) => f.apply(all);
+
+  test('an empty filter keeps everything', () {
+    const filter = LibraryFilter();
+
+    expect(filter.isActive, isFalse);
+    expect(apply(filter), all);
+  });
+
+  test('search matches every word of the query anywhere in the name, ignoring case', () {
+    expect(apply(const LibraryFilter(query: 'squat')), [squat]);
+    expect(apply(const LibraryFilter(query: 'barbell SQU')), [squat]);
+    expect(apply(const LibraryFilter(query: '  dead  romanian ')), [deadlift]);
+    expect(apply(const LibraryFilter(query: 'curl squat')), isEmpty);
+  });
+
+  test('choices within one filter widen, filters together narrow', () {
+    expect(apply(const LibraryFilter(muscles: {'biceps', 'hamstrings'})), [curl, deadlift]);
+    expect(apply(const LibraryFilter(muscles: {'biceps', 'hamstrings'}, equipment: {'barbell'})), [deadlift]);
+    expect(apply(const LibraryFilter(levels: {'expert', 'intermediate'})), [curl, deadlift]);
+  });
+
+  test('the muscle filter looks at the primary muscles only', () {
+    expect(apply(const LibraryFilter(muscles: {'glutes'})), [mystery]);
+  });
+
+  test('an Exercise without equipment drops out once equipment is filtered', () {
+    expect(apply(const LibraryFilter(equipment: {'barbell', 'dumbbell', 'body only'})), [squat, curl, deadlift]);
+  });
+
+  test('toggling a choice adds and removes it, and clearing keeps the query', () {
+    final filter = const LibraryFilter(query: 'curl').toggleMuscle('biceps').toggleLevel('beginner');
+
+    expect(filter.muscles, {'biceps'});
+    expect(filter.toggleMuscle('biceps').muscles, isEmpty);
+    expect(filter.activeChoices, 2);
+    expect(filter.clearChoices(), const LibraryFilter(query: 'curl'));
+  });
+
+  test('the options are the values present, levels in their natural order', () {
+    final options = FilterOptions.of(all);
+
+    expect(options.muscles, ['biceps', 'glutes', 'hamstrings', 'quadriceps']);
+    expect(options.equipment, ['barbell', 'dumbbell']);
+    expect(options.levels, ['beginner', 'intermediate', 'expert']);
+  });
+}