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>

autoload/exercise_engine.gd

@@ -34,6 +34,16 @@ func get_missions() -> Array[Dictionary]:
     return missions
 
 
+## Missions dont la regle appartient au niveau scolaire demande (le niveau
+## d'une mission est celui de sa regle, pas un champ de plus a tenir a jour).
+func get_missions_for_level(level: String) -> Array[Dictionary]:
+    var missions: Array[Dictionary] = []
+    for mission in get_missions():
+        if get_rule(mission["rule_id"]).get("level", "") == level:
+            missions.append(mission)
+    return missions
+
+
 func get_mission(mission_id: String) -> Dictionary:
     return _missions.get(mission_id, {})