Prototype governance vote attribution

[ericmann]

Summary

This is a draft prototype for review under Canton Development Fund PR canton-network#223, Milestone 1: Governance-Voting Identity and CIP. It builds on the governance-voter taxonomy branch and is intended for fork review before any future upstream discussion; it is not final upstream design.

• Adds VoteCastRole and explicit castBy / castByRole fields to Vote.
• Records existing operator-cast votes as VCR_Operator while preserving represented-SV vote slots and trackingCid behavior.
• Updates Daml helpers/tests so vote updates continue to preserve one vote per represented SV while retaining operator attribution.
• Extends the governance-voter prototype docs with the attribution/tally distinction.

Design IDs: GV-001, GV-003, GV-004, GV-006.

Test Plan

• direnv exec . sbt "splice-dso-governance-test-daml/damlTest"
• direnv exec . sbt damlDarsLockFileUpdate

[ericmann]

Superseded by consolidated draft PR #6 for single-thread review.