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/ui/variety_detail_screen_test.dart

@@ -157,4 +157,42 @@ void main() {
       await disposeTree(tester);
     },
   );
+
+  testWidgets('recording a germination test shows the rate badge on the lot', (
+    tester,
+  ) async {
+    final repo = newTestRepository(db);
+    final id = await repo.addQuickVariety(label: 'Maize');
+    await repo.addLot(
+      varietyId: id,
+      harvestYear: 2024,
+      quantity: const Quantity(kind: QuantityKind.cob),
+    );
+
+    await tester.pumpWidget(
+      wrapDetail(
+        repository: repo,
+        varietyId: id,
+        species: newTestSpeciesRepository(db),
+      ),
+    );
+    await tester.pumpAndSettle();
+
+    await tester.tap(find.byIcon(Icons.grass_outlined).first);
+    await tester.pumpAndSettle();
+
+    await tester.enterText(
+      find.byKey(const Key('germination.germinated')),
+      '18',
+    );
+    await tester.enterText(
+      find.byKey(const Key('germination.sampleSize')),
+      '20',
+    );
+    await tester.tap(find.byKey(const Key('germination.save')));
+    await tester.pumpAndSettle();
+
+    expect(find.text('90%'), findsOneWidget); // germination badge on the lot
+    await disposeTree(tester);
+  });
 }