Add the playable scene loop: boot through mission to reward

Wires every autoload up to a real UI: boot -> profile select/create
-> world map (three mission markers, positioned from data, not
hardcoded pixels) -> mission intro (narrative from data/missions/,
a new data type with its own schema) -> rule panel -> exercise host
-> one of the three exercise type scenes -> feedback panel -> reward
screen -> back to the map.

exercise_host is the only piece with real orchestration logic: picks
exercises via ExerciseEngine (now fully implemented, validating at
load like make validate-data does), validates answers through the
pure AnswerValidator/DictationAnalyzer, escalates aid over 3 attempts
via AttemptFlow, feeds DifficultyManager so adaptive difficulty is
actually wired in rather than dead code, and records every attempt
through ProgressManager. Every exercise-type scene (multiple_choice,
sentence_correction, dictation) only emits its answer by signal, per
the architecture invariant.

All child-facing text now comes from data/localization/fr.json via
UIText, not hardcoded in scripts or scenes. Getting that right
required extending SchemaValidator to support "additionalProperties"
as a schema (dynamic keys), which also closed a real gap: save.skills
was previously unvalidated against SkillState's actual shape.

16 new headless tests, including two integration tests that instance
exercise_host.tscn directly and drive it by emitting child-scene
signals, stopping short of the final scene transition (which would
tear down the test runner's own tree). Everything that needs a
screen -- layout at different sizes, touch, virtual keyboard, actual
audio playback (no .ogg files exist yet) -- is listed in the new
docs/testing/manual-checklist.md instead of claimed as verified.

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

scripts/data/validate_data_cli.gd

@@ -12,6 +12,8 @@ func _init() -> void:
     checked += _validate_simple_dir("res://data/curriculum", "curriculum", failures)
     checked += _validate_dictations("res://data/dictations", failures)
     checked += _validate_exercises("res://data/exercises", failures)
+    checked += _validate_missions("res://data/missions", failures)
+    checked += _validate_simple_dir("res://data/localization", "localization", failures)
 
     if failures.is_empty():
         print("validate-data : %d fichier(s) JSON valide(s)" % checked)
@@ -43,6 +45,30 @@ func _validate_dictations(dir_path: String, failures: Array[String]) -> int:
     return count
 
 
+func _validate_missions(dir_path: String, failures: Array[String]) -> int:
+    var known_rule_ids: Array[String] = []
+    for path in DataLoader.find_json_files("res://data/rules"):
+        var rule: Variant = DataLoader.read_json(path)
+        if typeof(rule) == TYPE_DICTIONARY and rule.has("rule_id"):
+            known_rule_ids.append(rule["rule_id"])
+
+    var count := 0
+    for path in DataLoader.find_json_files(dir_path):
+        count += 1
+        var schema_errors := DataLoader.validate_file(path, "mission")
+        failures.append_array(schema_errors)
+        if not schema_errors.is_empty():
+            continue
+        var data: Dictionary = DataLoader.read_json(path)
+        if not known_rule_ids.has(data["rule_id"]):
+            var message := (
+                "%s : rule_id '%s' ne correspond a aucune regle de data/rules"
+                % [path, data["rule_id"]]
+            )
+            failures.append(message)
+    return count
+
+
 func _validate_exercises(dir_path: String, failures: Array[String]) -> int:
     var count := 0
     for path in DataLoader.find_json_files(dir_path):