Prevent one-shot sound stacking on retriggered presses

[vanlueckn]

Summary

• Fixes sound stacking when a cab control is pressed repeatedly without releasing
• When a one-shot retriggered, the old voice is now faded out over 15ms instead of stopping abruptly
• Reduces loudness buildup from repeated key presses (e.g. ten presses ≠ ten times the level)
• Introduces STEAL tween constant for retriggered one-shot fadeout
• Stores last played handle per source in Audio::shots HashMap to enable voice replacement

Testing

• Rapid repeated presses of any cab control (button, switch, lever) that triggers one-shots — verify no volume accumulation
• Single press of a control — verify no audible change to timing or quality
• Audio output with rapid manual retriggering in simulator — confirm smooth continuity without clicks or pops