[Developer] #204 Phase 1 MVP — Flutter app skeleton complete

- Drift 21 tables (8 catalog + 11 user + habit_dose_variants + meta_kv)
  with R1~R10 CHECK constraints and 19 indexes
- 8 hand-crafted seed JSON catalogs in app/assets/seed/
  (refs 84, protocols 34, methodologies 21, frame_patterns 30,
   reward_menu_items 30, break_protocols 8, common_frames 5, diet_patterns 5)
- 6 domain functions: recommend_variant, compute_streak,
  validate_frame_level, active_habit_quota, weekly_minimum_ratio,
  seed_importer (transactional, idempotent)
- 4 vertical-slice Riverpod screens: HabitList, HabitCreate, CheckIn, Streak
- 31 unit tests passing; flutter analyze clean
- OQ-5 streak semantics: missing entry ≠ explicit blank
  (missing = end of history; only TrackerValue.blank triggers Never-miss-twice)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/lib/domain/rules/active_habit_quota.dart

@@ -0,0 +1,42 @@
+import '../models/habit.dart';
+
+/// R1/R2: max active habits per type.
+/// - build: ≤ 3
+/// - break: ≤ 1
+const int kMaxActiveBuild = 3;
+const int kMaxActiveBreak = 1;
+
+class QuotaResult {
+  final HabitType type;
+  final int currentCount;
+  final int limit;
+  final bool allowed;
+
+  const QuotaResult({
+    required this.type,
+    required this.currentCount,
+    required this.limit,
+    required this.allowed,
+  });
+
+  String get reason {
+    if (allowed) return 'ok';
+    return type == HabitType.build
+        ? 'build habit quota reached (≤ $kMaxActiveBuild active)'
+        : 'break habit quota reached (≤ $kMaxActiveBreak active)';
+  }
+}
+
+/// Pure judgment: caller passes in the current active count.
+QuotaResult judgeActiveHabitQuota({
+  required HabitType type,
+  required int currentActiveCount,
+}) {
+  final limit = type == HabitType.build ? kMaxActiveBuild : kMaxActiveBreak;
+  return QuotaResult(
+    type: type,
+    currentCount: currentActiveCount,
+    limit: limit,
+    allowed: currentActiveCount < limit,
+  );
+}