Add project skeleton: Godot setup, tooling, and smoke test

Pins Godot 4.6.3-stable and GUT 9.6.0 (9.6.1+ reference
AccessibilityServer, unavailable in 4.6.3) via tools/setup.sh. Wires
make check (gdlint + gdformat config for 4-space indent, headless
GUT run, and a JSON well-formedness check ahead of real schema
validation). Seven autoload stubs match the architecture in the
cahier des charges; real logic lands with the following steps.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

scripts/data/validate_data_cli.gd

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+extends SceneTree
+## Outil headless invoque par `make validate-data`.
+## A ce stade, verifie seulement que les fichiers data/**/*.json sont du JSON bien forme.
+## La validation contre les schemas (rule, exercise, dictation, curriculum, save) sera
+## ajoutee a l'etape "Donnees" (voir Ordre de construction, docs/specs/decisions.md point 2).
+
+
+func _init() -> void:
+    var json_paths: Array[String] = _find_json_files("res://data")
+    var failures: Array[String] = []
+    for path in json_paths:
+        var file := FileAccess.open(path, FileAccess.READ)
+        if file == null:
+            failures.append("%s : lecture impossible" % path)
+            continue
+        var text := file.get_as_text()
+        if text.strip_edges().is_empty():
+            continue
+        if JSON.parse_string(text) == null:
+            failures.append("%s : JSON invalide" % path)
+
+    if failures.is_empty():
+        print("validate-data : %d fichier(s) JSON valide(s)" % json_paths.size())
+        quit(0)
+    else:
+        for failure in failures:
+            printerr(failure)
+        quit(1)
+
+
+func _find_json_files(root_path: String) -> Array[String]:
+    var result: Array[String] = []
+    var dir := DirAccess.open(root_path)
+    if dir == null:
+        return result
+    dir.list_dir_begin()
+    var entry := dir.get_next()
+    while entry != "":
+        if entry != "." and entry != "..":
+            var full_path := root_path.path_join(entry)
+            if dir.current_is_dir():
+                result.append_array(_find_json_files(full_path))
+            elif entry.ends_with(".json"):
+                result.append(full_path)
+        entry = dir.get_next()
+    return result

scripts/data/validate_data_cli.gd.uid

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+uid://01g5hgfcb5tq