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/schemas/curriculum.schema.json

@@ -0,0 +1,24 @@
+{
+  "type": "object",
+  "required": [
+    "level",
+    "region_id",
+    "chapter_id",
+    "title",
+    "rule_ids",
+    "review_status"
+  ],
+  "additionalProperties": false,
+  "properties": {
+    "level": { "type": "string", "enum": ["CE1", "CE2"] },
+    "region_id": { "type": "string", "pattern": "^[a-z][a-z0-9_]*$" },
+    "chapter_id": { "type": "string", "pattern": "^[a-z][a-z0-9_]*$" },
+    "title": { "type": "string", "minLength": 1 },
+    "rule_ids": {
+      "type": "array",
+      "minItems": 1,
+      "items": { "type": "string", "pattern": "^[a-z][a-z0-9_]*$" }
+    },
+    "review_status": { "type": "string", "enum": ["draft", "validated"] }
+  }
+}