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>

tests/data/test_schema_validator.gd

@@ -0,0 +1,69 @@
+extends GutTest
+## Tests du validateur de schema generique (scripts/data/schema_validator.gd),
+## independants de tout contenu reel de data/.
+
+var schema: Dictionary = {
+    "type": "object",
+    "required": ["id", "level"],
+    "additionalProperties": false,
+    "properties": {
+        "id": { "type": "string", "pattern": "^[a-z][a-z0-9_]*$" },
+        "level": { "type": "string", "enum": ["CE1", "CE2"] },
+        "difficulty": { "type": "integer", "minimum": 1, "maximum": 3 },
+        "tags": { "type": "array", "minItems": 1, "items": { "type": "string" } },
+    },
+}
+
+
+func test_valid_object_has_no_errors() -> void:
+    var data := { "id": "ex_01", "level": "CE1", "difficulty": 2, "tags": ["pluriel"] }
+    assert_eq(SchemaValidator.validate(data, schema), [])
+
+
+func test_missing_required_property_is_reported() -> void:
+    var data := { "level": "CE1" }
+    var errors := SchemaValidator.validate(data, schema)
+    assert_eq(errors.size(), 1)
+    assert_string_contains(errors[0], "id")
+
+
+func test_wrong_type_is_reported() -> void:
+    var data := { "id": "ex_01", "level": "CE1", "difficulty": "deux" }
+    var errors := SchemaValidator.validate(data, schema)
+    assert_eq(errors.size(), 1)
+    assert_string_contains(errors[0], "difficulty")
+
+
+func test_enum_violation_is_reported() -> void:
+    var data := { "id": "ex_01", "level": "CM1" }
+    var errors := SchemaValidator.validate(data, schema)
+    assert_eq(errors.size(), 1)
+    assert_string_contains(errors[0], "level")
+
+
+func test_pattern_violation_is_reported() -> void:
+    var data := { "id": "Ex-01", "level": "CE1" }
+    var errors := SchemaValidator.validate(data, schema)
+    assert_eq(errors.size(), 1)
+    assert_string_contains(errors[0], "id")
+
+
+func test_unexpected_property_is_reported_when_additional_forbidden() -> void:
+    var data := { "id": "ex_01", "level": "CE1", "extra": true }
+    var errors := SchemaValidator.validate(data, schema)
+    assert_eq(errors.size(), 1)
+    assert_string_contains(errors[0], "extra")
+
+
+func test_min_items_violation_is_reported() -> void:
+    var data := { "id": "ex_01", "level": "CE1", "tags": [] }
+    var errors := SchemaValidator.validate(data, schema)
+    assert_eq(errors.size(), 1)
+    assert_string_contains(errors[0], "tags")
+
+
+func test_numeric_bounds_are_checked() -> void:
+    var data := { "id": "ex_01", "level": "CE1", "difficulty": 9 }
+    var errors := SchemaValidator.validate(data, schema)
+    assert_eq(errors.size(), 1)
+    assert_string_contains(errors[0], "difficulty")