Show "Exercise N/3" so repeated content doesn't look like a freeze

Manual report: after "Bravo!", clicking Continue seemed to do
nothing. An automated test proved the mission does advance (a new
exercise instance loads, completed_count increments) -- but
mission_02_pont_des_phrases has exactly one sentence_correction
exercise in the data right now, so the "next" exercise is the
identical sentence the child just solved. Nothing was broken; there
was just no visible signal that progress happened.

Added a persistent progress label at the top of exercise_host,
updated on every exercise load. Doesn't address the underlying
content-volume gap (still tracked in decisions.md point 14), but
makes the repetition legible instead of indistinguishable from a
stall.

Rebuilt both exports.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

scenes/exercises/exercise_host.gd

@@ -26,6 +26,7 @@ var _last_exercise_id := ""
 
 @onready var _exercise_container: Control = %ExerciseContainer
 @onready var _overlay_container: Control = %OverlayContainer
+@onready var _progress_label: Label = %ProgressLabel
 
 
 ## Accesseurs reserves aux tests d'integration (scenes/exercises/
@@ -67,6 +68,7 @@ func _load_next_exercise() -> void:
         _mission, _last_exercise_id, _current_difficulty
     )
     _attempt_number = 1
+    _update_progress_label()
 
     if _current_exercise_scene != null:
         _current_exercise_scene.queue_free()
@@ -78,6 +80,15 @@ func _load_next_exercise() -> void:
     _connect_answer_signal()
 
 
+## Un contenu encore peu nombreux (voir docs/specs/decisions.md, point 14)
+## peut faire redonner le meme exercice d'une mission a l'autre tentative :
+## sans ce repere, ca ressemble a un blocage plutot qu'a une progression.
+func _update_progress_label() -> void:
+    _progress_label.text = "%s %d/%d" % [
+        UIText.get_text("exercise_host.progress_label"), _completed_count + 1, MISSION_LENGTH
+    ]
+
+
 func _connect_answer_signal() -> void:
     match _mission["exercise_type"]:
         "multiple_choice":