Add loop playback support

[grimmdude]

Summary

• Adds a loop property to the Player class — when true, playback restarts automatically at end-of-file instead of stopping
• Marks the internal playLoop() method as @private in JSDoc to clarify it is not a public looping API
• The endOfFile event is still emitted on each loop iteration so consumers can react

Usage

const player = new Player();
player.loop = true;
player.loadFile('song.mid');
player.play(); // restarts automatically at end

Closes #105

Test plan

• All existing tests pass (42 passing)
• Manual test: play a MIDI file with loop = false (default) — should stop at end
• Manual test: play a MIDI file with loop = true — should restart seamlessly at end
• Verify endOfFile event fires on each loop iteration

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