Add CE1 board 3 and CE2 boards 2 and 3 with new backdrops (0.7.0)

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

data/curriculum/ce2_lettres_et_mots.json

@@ -0,0 +1,15 @@
+{
+  "level": "CE2",
+  "region_id": "ile_des_lettres",
+  "chapter_id": "lettres_et_mots",
+  "title": "Les lettres et les mots",
+  "region_title": "L'île des lettres",
+  "order": 3,
+  "backdrop": "island",
+  "rule_ids": [
+    "regle_feminin_noms_adjectifs",
+    "regle_g_ge_gu",
+    "regle_lettre_muette_finale"
+  ],
+  "review_status": "draft"
+}