ship a sample bus layout with Music and FX

Adds default_bus_layout.tres at the addon root containing a Music bus and an
FX bus, each routing into Master. Users can point Project Settings → Audio →
Default Bus Layout at this file (or copy it to their project root) to get the
buses the players auto-route to in the editor. README documents the wiring.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

default_bus_layout.tres

@@ -0,0 +1,15 @@
+[gd_resource type="AudioBusLayout" format=3]
+
+[resource]
+bus/1/name = &"Music"
+bus/1/solo = false
+bus/1/mute = false
+bus/1/bypass_fx = false
+bus/1/volume_db = 0.0
+bus/1/send = &"Master"
+bus/2/name = &"FX"
+bus/2/solo = false
+bus/2/mute = false
+bus/2/bypass_fx = false
+bus/2/volume_db = 0.0
+bus/2/send = &"Master"