feat(block1): germination tests per lot with derived rate

Record germination tests on a lot and surface the result.

- GerminationEntry model with a derived rate (germinated / sample). VarietyLot
  carries its tests (most-recent first) and exposes latestGerminationRate.
- VarietyRepository.addGerminationTest; watchVariety now also re-emits on
  germination-test changes, so the detail refreshes live.
- Detail UI: each lot shows a germination % badge and a grass action that opens
  a sheet listing past tests and adding a new one (germinated + sample size).
  i18n strings (ES/EN).

Tests: derived-rate + reactivity at the repository, and a widget test for the
record-test → badge flow. Full suite: 35 passing, 0 skipped.

apps/app_seeds/test/data/variety_detail_test.dart

@@ -97,4 +97,42 @@ void main() {
     final detail = await repo.watchVariety(id).first;
     expect(detail!.category, 'My tomatoes');
   });
+
+  test(
+    'addGerminationTest surfaces the latest derived rate on the lot',
+    () async {
+      final id = await repo.addQuickVariety(label: 'Maize');
+      final lotId = await repo.addLot(varietyId: id, harvestYear: 2024);
+      await repo.addGerminationTest(
+        lotId: lotId,
+        testedOn: 1000,
+        sampleSize: 20,
+        germinatedCount: 18,
+      );
+
+      final lot = (await repo.watchVariety(id).first)!.lots.single;
+      expect(lot.germinationTests, hasLength(1));
+      expect(lot.latestGerminationRate, closeTo(0.9, 1e-9));
+    },
+  );
+
+  test('watchVariety reacts to a germination test being added', () async {
+    final id = await repo.addQuickVariety(label: 'Maize');
+    final lotId = await repo.addLot(varietyId: id);
+    final queue = StreamQueue(repo.watchVariety(id));
+
+    final initial = await queue.next;
+    expect(initial!.lots.single.germinationTests, isEmpty);
+
+    await repo.addGerminationTest(
+      lotId: lotId,
+      sampleSize: 10,
+      germinatedCount: 9,
+    );
+
+    final updated = await queue.next;
+    expect(updated!.lots.single.latestGerminationRate, closeTo(0.9, 1e-9));
+
+    await queue.cancel();
+  });
 }