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>

data/exercises/ce1/ce1_accord_adjectif_correction_01.json

@@ -0,0 +1,25 @@
+{
+  "id": "ce1_accord_adjectif_correction_01",
+  "level": "CE1",
+  "domain": "grammaire",
+  "skill": "accord_groupe_nominal",
+  "rule_id": "regle_accord_adjectif",
+  "difficulty": 2,
+  "type": "sentence_correction",
+  "instruction": "Touche le mot qui contient une erreur, puis corrige-le.",
+  "tokens": [
+    { "text": "Les", "correct": true },
+    {
+      "text": "petit",
+      "correct": false,
+      "expected": "petits",
+      "rule_id": "regle_accord_adjectif"
+    },
+    { "text": "chiens", "correct": true },
+    { "text": "courent.", "correct": true }
+  ],
+  "hint": "Le determinant « les » indique un pluriel.",
+  "explanation": "L'adjectif « petit » s'accorde avec le nom « chiens », qui est au pluriel : il faut ecrire « petits ».",
+  "review_tags": ["pluriel", "accord_adjectif"],
+  "review_status": "draft"
+}

data/exercises/ce1/ce1_accord_adjectif_qcm_01.json

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+{
+  "id": "ce1_accord_adjectif_qcm_01",
+  "level": "CE1",
+  "domain": "grammaire",
+  "skill": "accord_groupe_nominal",
+  "rule_id": "regle_accord_adjectif",
+  "difficulty": 1,
+  "type": "multiple_choice",
+  "instruction": "Choisis le bon groupe de mots.",
+  "question": "Les ... chiens jouent.",
+  "answers": ["petit", "petits", "petite"],
+  "correct_answer": 1,
+  "hint": "Observe le determinant « les » et le nom « chiens ».",
+  "explanation": "Le groupe nominal est au pluriel. L'adjectif prend ici la marque du pluriel.",
+  "review_tags": ["pluriel", "accord_adjectif"],
+  "review_status": "draft"
+}

data/exercises/ce1/ce1_accord_adjectif_qcm_02.json

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+{
+  "id": "ce1_accord_adjectif_qcm_02",
+  "level": "CE1",
+  "domain": "grammaire",
+  "skill": "accord_groupe_nominal",
+  "rule_id": "regle_accord_adjectif",
+  "difficulty": 1,
+  "type": "multiple_choice",
+  "instruction": "Choisis le bon groupe de mots.",
+  "question": "La ... maison est jolie.",
+  "answers": ["petit", "petite", "petites"],
+  "correct_answer": 1,
+  "hint": "Observe le determinant « la » et le nom « maison ».",
+  "explanation": "Le nom « maison » est feminin singulier. L'adjectif prend donc la marque du feminin.",
+  "review_tags": ["feminin", "accord_adjectif"],
+  "review_status": "draft"
+}