Prototype governance voter vote taxonomy

[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 is intended to make the proposed Phase 1 vote taxonomy concrete for review and future upstream discussion; it is not final upstream design.

• Adds an explicit Daml allowlist for governance-voter eligible actions via isGovernanceVoterAction.
• Covers supported and unsupported action constructors in Daml tests, including the proposed SRARC_OffboardSv inclusion.
• Adds a regression test showing vote updates remain keyed by represented SV, preserving one-vote-per-SV semantics.
• Documents that the allowlist is proposed and subject to maintainer/CIP validation.

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Superseded by consolidated draft PR #6 for single-thread review.