Prototype governance voter cast path

[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 binding branch and is intended for fork review before any future upstream discussion; it is not final upstream design.

• Adds DsoRules_CastGovernanceVote using a SvGovernanceVoter contract ID rather than a key lookup.
• Validates binding dso, represented SV, governance-voter signer, signer role, and the governance-voter action allowlist.
• Records votes in the represented SV's existing vote slot, preserving current tally and trackingCid semantics.
• Covers eligible votes, unsupported actions, wrong/rotated/cleared voters, and operator/governance-voter overwrites in Daml tests.

Design IDs: GV-001, GV-002, 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Superseded by consolidated draft PR #6 for single-thread review.