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/curriculum/ce1_accords_groupe_nominal.json

@@ -0,0 +1,8 @@
+{
+  "level": "CE1",
+  "region_id": "foret_des_accords",
+  "chapter_id": "accords_groupe_nominal",
+  "title": "Les accords dans le groupe nominal",
+  "rule_ids": ["regle_accord_adjectif"],
+  "review_status": "draft"
+}