Never repeat an exercise within a mission; remove near-duplicate sentences (0.5.5)

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

scripts/data/exercise_picker.gd

@@ -23,3 +23,24 @@ static func pick(
         pool = candidates
 
     return pool[rng.randi_range(0, pool.size() - 1)]
+
+
+## Tire un exercice qui n'a pas encore ete vu pendant la mission : d'abord
+## parmi `preferred` (la difficulte visee), puis parmi `all` si tous les
+## preferes ont ete vus. Quand tout a ete vu (mission plus longue que le
+## contenu disponible), retombe sur `pick` en evitant au moins le dernier vu.
+static func pick_unseen(
+    preferred: Array[Dictionary],
+    all: Array[Dictionary],
+    seen_ids: Array[String],
+    rng: RandomNumberGenerator
+) -> Variant:
+    for source in [preferred, all]:
+        var pool: Array[Dictionary] = []
+        for candidate: Dictionary in source:
+            if not seen_ids.has(candidate["id"]):
+                pool.append(candidate)
+        if not pool.is_empty():
+            return pool[rng.randi_range(0, pool.size() - 1)]
+    var last := "" if seen_ids.is_empty() else seen_ids[seen_ids.size() - 1]
+    return pick(preferred, last, rng)