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/lib/i18n/en.i18n.json

@@ -1,4 +1,30 @@
 {
+  "seedSaving": {
+    "title": "Saving its seed",
+    "subtitle": "What it takes to keep the variety true",
+    "lifeCycle": "Cycle",
+    "cycleAnnual": "Annual",
+    "cycleBiennial": "Biennial — seeds in year 2",
+    "cyclePerennial": "Perennial",
+    "pollination": "Pollination",
+    "pollSelf": "Self-pollinating",
+    "pollCross": "Crosses with others",
+    "pollMixed": "Self-pollinates, sometimes crosses",
+    "byInsect": "by insects",
+    "byWind": "on the wind",
+    "isolation": "Keep apart",
+    "isolationRange": "{min}–{max} m from other varieties",
+    "isolationSingle": "{min} m from other varieties",
+    "plants": "Save from several plants",
+    "plantsValue": "From at least {n} plants",
+    "processing": "Cleaning the seed",
+    "procDry": "Dry seed (thresh)",
+    "procWet": "Wet seed (ferment and rinse)",
+    "difficulty": "Difficulty",
+    "diffEasy": "Easy",
+    "diffMedium": "Medium",
+    "diffHard": "Hard"
+  },
   "calendar": {
     "title": "This month",
     "filterChip": "This month",