Add sound effects, ambient music and volume mixing (0.5.0)

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

scripts/audio/sound_catalog.gd

@@ -0,0 +1,27 @@
+class_name SoundCatalog
+extends RefCounted
+## Identifiants des effets sonores et des musiques, resolus vers un fichier
+## d'un repertoire fixe (jamais de chemin libre : voir CLAUDE.md). Code pur.
+
+const SFX_DIR := "res://assets/audio/sfx"
+const MUSIC_DIR := "res://assets/audio/music"
+
+const SFX_IDS: Array[String] = ["click", "success", "try_again", "star", "fanfare"]
+const MUSIC_IDS: Array[String] = ["forest", "marsh"]
+
+
+static func sfx_path(sfx_id: String) -> String:
+    if not SFX_IDS.has(sfx_id):
+        return ""
+    return "%s/%s.ogg" % [SFX_DIR, sfx_id]
+
+
+static func music_path(music_id: String) -> String:
+    if not MUSIC_IDS.has(music_id):
+        return ""
+    return "%s/%s.ogg" % [MUSIC_DIR, music_id]
+
+
+## Musique d'une region selon le niveau scolaire de la regle.
+static func music_for_level(level: String) -> String:
+    return "marsh" if level == "CE2" else "forest"