Add CE2 homophones chapter (a/à, et/est, son/sont, on/ont)

Same directives as the CE1 chapter: 4 rules with a substitution test
each, 8 multiple-choice + 4 sentence corrections + 3 dictées per rule
(draft), 3 missions per rule, Piper audio for the 12 dictées.

21 missions don't fit on one map, so the world map now shows the
missions of the profile's school level (derived from the rule's level,
no data-format change), with a button letting a CE2 profile revisit the
CE1 map. Mission-position guard test now checks each level separately.

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

data/missions/mission_15_source_chantante.json

@@ -0,0 +1,12 @@
+{
+  "mission_id": "mission_15_source_chantante",
+  "title": "La source chantante",
+  "narrative_context": "La source chante une phrase, une seule fois. Écoute-la bien et écris exactement ce qu'elle chante.",
+  "rule_id": "regle_homophone_et_est",
+  "exercise_type": "dictation",
+  "map_position": {
+    "x": 0.375,
+    "y": 0.85
+  },
+  "review_status": "draft"
+}