code: Clarify and improve handling of multiple proposals for same height/round

[romac]

Summary

There's currently ambiguity around how multiple proposals for the same height/round should be handled, particularly in cases of Byzantine proposers. The current implementation has the application discard subsequent proposals after the first one, but this needs to be documented and potentially revised.

Action items

• Document current behavior regarding proposal handling in tutorials and doc comments
• Document the need for ValueSync in scenarios with a Byzantine proposer
• Consider implementing support for multiple proposals per height/round:
  • Review existing partial implementation
  • Design test cases for Byzantine scenarios
  • Implement changes to allow consensus to store multiple proposal values