Add replaceInstrument() to Plugin API

[manolo]

Resolves: #31201

Add a new Score.replaceInstrument(part, instrumentId) method to the Plugin API, allowing plugins to programmatically change instruments on score parts.

Changes

• Add Q_INVOKABLE Score::replaceInstrument(Part*, QString) to the plugin API
• Bridge plugin API to existing NotationParts::replaceInstrument() infrastructure
• Use Part::MAIN_INSTRUMENT_TICK to correctly identify the main instrument vs mid-score instrument changes
• Add unit tests for instrument replacement with undo/redo support
• Add test environment setup for API tests

Usage Example

var part = curScore.parts[0];
curScore.replaceInstrument(part, "violin");

The instrumentId parameter should match IDs from instruments.xml.

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