[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/core/result.dart

@@ -0,0 +1,16 @@
+/// Minimal Result sum type. Use for domain operations that may fail.
+sealed class Result<T, E> {
+  const Result();
+  bool get isOk => this is Ok<T, E>;
+  bool get isErr => this is Err<T, E>;
+}
+
+final class Ok<T, E> extends Result<T, E> {
+  final T value;
+  const Ok(this.value);
+}
+
+final class Err<T, E> extends Result<T, E> {
+  final E error;
+  const Err(this.error);
+}