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>

CLAUDE.md

@@ -16,6 +16,7 @@ Le `Makefile` est le seul point d'entrée pour les opérations système. Tenir c
 - `make validate-data` : valide tous les JSON de `data/` contre leurs schémas
 - `make test` : tests en mode headless
 - `make check` : lint + validate-data + test (à passer avant chaque commit)
+- `make content-review` : régénère `docs/review/content-review.md` à partir de `data/`
 - `make export-linux` / `make export-windows` : exports dans `build/`
 
 ## Conventions de code