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.tscn

@@ -11,12 +11,27 @@ grow_horizontal = 2
 grow_vertical = 2
 script = ExtResource("1")
 
+[node name="HeaderBar" type="MarginContainer" parent="."]
+layout_mode = 1
+anchor_right = 1.0
+offset_bottom = 40.0
+grow_horizontal = 2
+theme_override_constants/margin_left = 16
+theme_override_constants/margin_top = 8
+theme_override_constants/margin_right = 16
+theme_override_constants/margin_bottom = 8
+
+[node name="ProgressLabel" type="Label" parent="HeaderBar"]
+unique_name_in_owner = true
+layout_mode = 2
+horizontal_alignment = 2
+
 [node name="ExerciseContainer" type="Control" parent="."]
 unique_name_in_owner = true
 layout_mode = 1
-anchors_preset = 15
 anchor_right = 1.0
 anchor_bottom = 1.0
+offset_top = 40.0
 grow_horizontal = 2
 grow_vertical = 2