feat(species): derived verified reference links (GBIF/Wikipedia/Wikispecies)

Add a read-only "Learn more" section on the variety detail that surfaces
verified external references for the linked species, built deterministically
from bundled identifiers — no titles bundled, no network, offline-first:

- GBIF taxon page from the backbone key
- Wikipedia via Wikidata Special:GoToLinkedPage (locale-aware, RTL-safe,
  falls back to the Wikidata item when a language lacks the article)
- Wikispecies from the scientific name

Enrich the bundled catalog (species.json v3) with gbif_key + wikidata_qid for
all 14 species, cross-checked (each QID carries GBIF ID P846 = the key).
Schema already had the columns; seedBundled now persists and backfills them.

The grower's own manual links stay but are demoted (progressive disclosure):
the section only shows when links exist and "Add link" is a low-emphasis
button, since hand-adding a link is the rarer power move.

Pure-Dart link builder is unit-tested; repo persistence/backfill and the
detail-screen section are covered too.

apps/app_seeds/test/ui/variety_detail_screen_test.dart

@@ -373,6 +373,56 @@ void main() {
     await disposeTree(tester);
   });
 
+  testWidgets(
+    'a linked species with identifiers shows verified reference links',
+    (tester) async {
+      final repo = newTestRepository(db);
+      final species = newTestSpeciesRepository(db);
+      await species.seedBundled(const [
+        SpeciesSeed(
+          scientificName: 'Solanum lycopersicum',
+          gbifKey: 2930137,
+          wikidataQid: 'Q23501',
+        ),
+      ]);
+      final match = (await species.search('Solanum')).single;
+      final id = await repo.addQuickVariety(label: 'Tomato');
+      await repo.linkSpecies(id, match.id);
+
+      await tester.pumpWidget(
+        wrapDetail(repository: repo, varietyId: id, species: species),
+      );
+      await tester.pumpAndSettle();
+
+      // The "Learn more" section renders one chip per available source.
+      expect(find.text('Learn more'), findsOneWidget);
+      expect(find.byKey(const Key('ref.gbif')), findsOneWidget);
+      expect(find.byKey(const Key('ref.wikipedia')), findsOneWidget);
+      expect(find.byKey(const Key('ref.wikispecies')), findsOneWidget);
+      await disposeTree(tester);
+    },
+  );
+
+  testWidgets('a variety with no species shows no reference section', (
+    tester,
+  ) async {
+    final repo = newTestRepository(db);
+    final id = await repo.addQuickVariety(label: 'Mystery seed');
+
+    await tester.pumpWidget(
+      wrapDetail(
+        repository: repo,
+        varietyId: id,
+        species: newTestSpeciesRepository(db),
+      ),
+    );
+    await tester.pumpAndSettle();
+
+    expect(find.text('Learn more'), findsNothing);
+    expect(find.byKey(const Key('ref.gbif')), findsNothing);
+    await disposeTree(tester);
+  });
+
   testWidgets('plant lots show the Plants chip and hide germination', (
     tester,
   ) async {