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_semantic_checks.gd

@@ -0,0 +1,45 @@
+extends GutTest
+## Tests des contraintes croisees (scripts/data/semantic_checks.gd).
+
+
+func test_multiple_choice_rejects_out_of_bounds_correct_answer() -> void:
+    var data := { "answers": ["a", "b"], "correct_answer": 5 }
+    var errors := SemanticChecks.check_multiple_choice(data, "test")
+    assert_eq(errors.size(), 1)
+
+
+func test_multiple_choice_accepts_valid_index() -> void:
+    var data := { "answers": ["a", "b"], "correct_answer": 1 }
+    assert_eq(SemanticChecks.check_multiple_choice(data, "test"), [])
+
+
+func test_sentence_correction_requires_at_least_one_error_token() -> void:
+    var data := { "tokens": [{ "text": "Les", "correct": true }] }
+    var errors := SemanticChecks.check_sentence_correction(data, "test")
+    assert_eq(errors.size(), 1)
+
+
+func test_sentence_correction_requires_expected_on_error_tokens() -> void:
+    var data := { "tokens": [{ "text": "petit", "correct": false }] }
+    var errors := SemanticChecks.check_sentence_correction(data, "test")
+    assert_eq(errors.size(), 1)
+    assert_string_contains(errors[0], "expected")
+
+
+func test_dictation_word_count_must_match_target_text() -> void:
+    var data := {
+        "target_text": "Les petits chats dorment.",
+        "words": [{ "text": "Les" }, { "text": "petits" }],
+    }
+    var errors := SemanticChecks.check_dictation(data, "test")
+    assert_eq(errors.size(), 1)
+
+
+func test_dictation_accepts_matching_word_count() -> void:
+    var data := {
+        "target_text": "Les petits chats dorment.",
+        "words": [
+            { "text": "Les" }, { "text": "petits" }, { "text": "chats" }, { "text": "dorment." }
+        ],
+    }
+    assert_eq(SemanticChecks.check_dictation(data, "test"), [])