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/dictations/ce1_accord_adjectif_dictee_01.json

@@ -0,0 +1,18 @@
+{
+  "id": "ce1_accord_adjectif_dictee_01",
+  "level": "CE1",
+  "domain": "grammaire",
+  "rule_ids": ["regle_accord_adjectif"],
+  "difficulty": 1,
+  "target_text": "Les petits chats dorment.",
+  "words": [
+    { "text": "Les", "expects_capital": true },
+    { "text": "petits", "rule_id": "regle_accord_adjectif" },
+    { "text": "chats" },
+    { "text": "dorment." }
+  ],
+  "audio_id": "ce1_accord_adjectif_dictee_01",
+  "audio_provisional": true,
+  "accepted_variants": [],
+  "review_status": "draft"
+}