docs: describe MusicalInstruments repeat-note fix and coverage gate

[ryanbarlow97]

Documents TF-Minecraft/MusicalInstruments#20:

• Playing a note cancels the slot change so the server stays on slot 9, which lets the same note repeat on Paper. Slot changes other plugins cancel are ignored. Slot 9 is the reset slot and never plays a note; configs that map it are warned about.
• Removes the description of the per-player repeating task, which the plugin no longer runs, and updates the architecture tree and class diagram.
• /instruments give ignores case and drops the item when the inventory is full. v.minecraft:<material> resolves, and items that resolve to air are skipped.
• Build section covers the 100% JaCoCo coverage gate and its model-data limitation.
• InstrumentPlayEvent is not fired for slot changes another plugin cancelled.

Updates projects/MusicalInstruments/overview.md and README.md. The script unit tests and check-indexes.py pass.

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