feat(seed-saving): per-crop 'how to save this seed' guidance

A bundled, curated knowledge layer — the differentiator of a serious
seed app. For a variety's crop it shows how the plant reproduces and what
it takes to keep the variety true: life cycle, pollination (self/cross +
insect/wind), isolation distance, how many plants to save from, dry/wet
processing, a difficulty pill, and a short tip — all in human words.

- assets/catalog/seed_saving.json: curated public-domain data (Seed to
  Seed / Seed Savers), keyed by botanical FAMILY (defaults) with
  per-species/genus OVERRIDES that win — so e.g. Vicia faba is correctly
  insect-cross-pollinated despite the selfing Fabaceae default, and maize
  is wind-pollinated with a large population. Notes are locale-keyed
  (es/en), extensible to any language — a starter set, not a ceiling.
- domain/seed_saving.dart (pure): enums + SeedSavingGuide (merge, note
  fallback) + SeedSavingData.guideFor (species → genus → family).
- data/seed_saving_catalog.dart: loads the asset once into an in-memory
  singleton (no DB table, no migration); injector loads it at startup.
- VarietyDetail gains  (from the linked species — the reliable
  key, vs the editable category); detail screen shows a _SeedSavingView
  when a guide resolves.
- i18n seedSaving block (labels + enum values + params) in en/es/pt/ast.
- Tests: domain lookup/merge, the real asset's key corrections, and the
  detail section shows/hides. 314 suite tests green; analyze clean.

apps/app_seeds/test/domain/seed_saving_test.dart

@@ -0,0 +1,67 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:tane/domain/seed_saving.dart';
+
+/// Pure lookup/merge logic for the seed-saving guidance: a per-species entry
+/// overrides its family default, a bare genus is a fallback, and unknown crops
+/// resolve to null.
+void main() {
+  final data = parseSeedSaving('''
+  {
+    "families": {
+      "Fabaceae": { "lifeCycle": "annual", "pollination": "self", "processing": "dry", "difficulty": "easy" },
+      "Solanaceae": { "pollination": "self", "isolationMinM": 3, "isolationMaxM": 15 }
+    },
+    "species": {
+      "Vicia faba": { "pollination": "cross", "vector": "insect", "isolationMinM": 180, "note": {"es": "aísla", "en": "isolate"} },
+      "Cucurbita": { "pollination": "cross", "vector": "insect" }
+    }
+  }
+  ''');
+
+  test('a species entry overrides its family default', () {
+    final g = data.guideFor(scientificName: 'Vicia faba', family: 'Fabaceae');
+    expect(g, isNotNull);
+    // Family said self+easy+annual; the species override flips pollination.
+    expect(g!.pollination, Pollination.cross);
+    expect(g.vector, PollinationVector.insect);
+    expect(g.isolationMinM, 180);
+    // Fields the species didn't set fall through to the family.
+    expect(g.lifeCycle, LifeCycle.annual);
+    expect(g.difficulty, SavingDifficulty.easy);
+    expect(g.processing, SeedProcessing.dry);
+  });
+
+  test('a bare genus key is a fallback when the species is unlisted', () {
+    final g = data.guideFor(scientificName: 'Cucurbita maxima', family: null);
+    expect(g?.pollination, Pollination.cross);
+    expect(g?.vector, PollinationVector.insect);
+  });
+
+  test('family-only lookup returns the family default', () {
+    final g = data.guideFor(family: 'Solanaceae');
+    expect(g?.pollination, Pollination.self);
+    expect(g?.isolationMaxM, 15);
+  });
+
+  test('lookup is case-insensitive', () {
+    expect(data.guideFor(family: 'fabaceae'), isNotNull);
+    expect(data.guideFor(scientificName: 'vicia faba'), isNotNull);
+  });
+
+  test('an unknown crop resolves to null', () {
+    expect(data.guideFor(scientificName: 'Musa acuminata', family: 'Musaceae'),
+        isNull);
+  });
+
+  test('noteFor falls back to English then any language', () {
+    final g = data.guideFor(scientificName: 'Vicia faba')!;
+    expect(g.noteFor('es'), 'aísla');
+    expect(g.noteFor('fr'), 'isolate'); // no fr → en
+  });
+
+  test('mergedWith unions notes with the override winning', () {
+    const base = SeedSavingGuide(note: {'es': 'a', 'en': 'b'});
+    const over = SeedSavingGuide(note: {'es': 'c'});
+    expect(base.mergedWith(over).note, {'es': 'c', 'en': 'b'});
+  });
+}