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/lib/i18n/es.i18n.json

@@ -35,6 +35,14 @@
     "year": "Año {year}",
     "noYear": "Año desconocido"
   },
+  "germination": {
+    "title": "Germinación",
+    "add": "Añadir prueba",
+    "sampleSize": "Muestra",
+    "germinated": "Germinadas",
+    "none": "Aún no hay pruebas de germinación.",
+    "result": "{percent}%"
+  },
   "editVariety": {
     "title": "Editar semilla",
     "name": "Nombre",