fix(musicxml gp-exporter): optimized handling of more than 4 voices

[Danielku15]

Issues

Fixes #2828

Proposed changes

Having more than 4 distinctive voices causes crashes in Guitar Pro. This PR updates two parts to optimize things:

• The exporter drops now voices if there are more than 4 filled ones. That should rarely be the case after the second fix but its a safety measure printing warnings.
• The MusicXML importer has now an optimized mechanism to assign the MusicXML voices to available voice slots. This brings quite significnat improvement for files where voices are introduced at various cases (additional staves also has new voices with high indices, individual notes are placed on new voices at some individual bars).

Most apps have rarely more than 4 voices and I also don't think many write more than four voices in a single track/staff.

Checklist

• I consent that this change becomes part of alphaTab under its current or any future open source license
• This PR is linked to an accepted issue (see above)
• Changes are implemented
• New tests were added
• I have read AGENTS.md if an AI helped draft any part of this PR

AI authorship disclosure

• No AI agent authored any part of this PR (description, code, tests, or commit messages)
• An AI agent contributed to this PR. The AI-authored disclosure block
  (alphatab-ai-authored-v1) is present at the top of this body, and I have
  personally reviewed every change and can explain each one

Further details

• This is a breaking change
• This change will require update of the documentation/website