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>

README.md

@@ -6,6 +6,21 @@ A Godot 4 audio addon for **dynamic music** (intensity-layered, seamless playlis
 
 1. Copy the `addons/gd-dynamic-sound/` folder into your project's `addons/` directory.
 2. Enable the plugin in **Project Settings → Plugins**.
+3. *(Optional, recommended)* Wire up the `Music` and `FX` buses — see below.
+
+## Audio buses
+
+When a player enters the tree in the editor, it auto-routes to a named bus if one exists:
+
+- `DynamicSoundPlayer` family → looks for a bus named **`Music`**
+- `DynamicSoundFXPlayer` family → looks for a bus named **`FX`**
+
+If the named bus doesn't exist, the player stays on `Master`. User-customised buses are never overridden — the auto-route only fires when `bus` is still the default `Master`.
+
+A ready-made layout containing both buses (each routing into `Master`) ships with the addon at `addons/gd-dynamic-sound/default_bus_layout.tres`. To use it:
+
+- **Whole-project replacement:** copy it to your project root as `default_bus_layout.tres`, or point Godot at the addon's copy via **Project Settings → General → Audio → Buses → Default Bus Layout**.
+- **Merge into an existing layout:** open Godot's audio bus panel and add `Music` and `FX` buses manually, both sending to `Master`.
 
 ## Concepts