Add JSON schemas, data validator, and first CE1 content

Introduces a generic JSON-Schema-subset validator in GDScript
(scripts/data/schema_validator.gd) used both by `make validate-data`
and, later, by ExerciseEngine at load time, plus semantic_checks.gd
for cross-field constraints a schema can't express (answer index
bounds, at least one faulty token in sentence correction, word count
matching between a dictation's target_text and its annotations).

Schemas cover rule, exercise (dispatched per type: multiple_choice,
sentence_correction), dictation, curriculum, and save (save.schema.json
is not yet wired into validate-data; saves live in user://, handled
at the Profiles/Saves step).

First draft content for the CE1 "accord_adjectif" rule (one rule, two
multiple-choice exercises, one sentence correction, one dictation,
one curriculum entry) exercises the full pipeline end to end and
seeds docs/review/content-review.md, regenerated by the new
tools/generate_content_review.py (`make content-review`).

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

scripts/data/data_loader.gd

@@ -0,0 +1,76 @@
+class_name DataLoader
+extends RefCounted
+## Charge et valide les fichiers JSON de data/ contre leurs schemas (data/schemas/).
+## Meme code utilise par `make validate-data` (headless) et par ExerciseEngine a
+## l'execution : une seule source de verite pour la validation (voir
+## docs/specs/decisions.md, point 2).
+
+const SCHEMA_DIR := "res://data/schemas"
+
+static var _schema_cache: Dictionary = {}
+
+
+## Mappe un type d'exercice (champ "type" des donnees) vers le nom de son schema.
+## Chaine vide si le type n'est pas reconnu : la liste des types est fermee ici,
+## pas devinee ailleurs.
+static func exercise_schema_name_for_type(exercise_type: String) -> String:
+    match exercise_type:
+        "multiple_choice":
+            return "exercise_multiple_choice"
+        "sentence_correction":
+            return "exercise_sentence_correction"
+        _:
+            return ""
+
+
+static func load_schema(schema_name: String) -> Dictionary:
+    if _schema_cache.has(schema_name):
+        return _schema_cache[schema_name]
+    var path := "%s/%s.schema.json" % [SCHEMA_DIR, schema_name]
+    var file := FileAccess.open(path, FileAccess.READ)
+    if file == null:
+        push_error("Schema introuvable : %s" % path)
+        return {}
+    var parsed: Variant = JSON.parse_string(file.get_as_text())
+    var schema: Dictionary = parsed if typeof(parsed) == TYPE_DICTIONARY else {}
+    _schema_cache[schema_name] = schema
+    return schema
+
+
+## Lit un fichier JSON et le valide contre le schema nomme. Retourne la liste des
+## erreurs (vide si valide) ; ne retourne jamais les donnees analysees, pour que
+## l'appelant ne puisse pas accidentellement utiliser une donnee non validee.
+static func validate_file(path: String, schema_name: String) -> Array[String]:
+    var parsed: Variant = read_json(path)
+    if parsed == null:
+        return ["%s : lecture impossible ou JSON invalide" % path]
+    var schema := load_schema(schema_name)
+    return SchemaValidator.validate(parsed, schema, path)
+
+
+static func read_json(path: String) -> Variant:
+    var file := FileAccess.open(path, FileAccess.READ)
+    if file == null:
+        return null
+    var text := file.get_as_text()
+    if text.strip_edges().is_empty():
+        return null
+    return JSON.parse_string(text)
+
+
+static func find_json_files(root_path: String) -> Array[String]:
+    var result: Array[String] = []
+    var dir := DirAccess.open(root_path)
+    if dir == null:
+        return result
+    dir.list_dir_begin()
+    var entry := dir.get_next()
+    while entry != "":
+        if entry != "." and entry != "..":
+            var full_path := root_path.path_join(entry)
+            if dir.current_is_dir():
+                result.append_array(find_json_files(full_path))
+            elif entry.ends_with(".json"):
+                result.append(full_path)
+        entry = dir.get_next()
+    return result