Add sound/music

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Current state

• No audio assets in assets/.
• Bounce SFX is generated procedurally in scripts/Ball.gd via AudioStreamWAV.
• No music system or menu/combat audio cues are implemented.

Suggested work

• Add SFX assets and route them through a shared audio manager/bus.
• Add a music plan (transitions, combat, elite, boss, shop, rest, menu music).
• Replace or complement the procedural bounce SFX with a sample if desired.

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Music system added with #114. working on composing music now.

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Elite/Boss themes still WIP