Prototype SV governance voter binding

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

• Adds an SV-declared SvGovernanceVoter binding template with sv as signatory and dso / governanceVoter as observers.
• Keeps the binding fetchable by contract ID with no contract key, leaving the one-active-binding-per-SV invariant for workflow validation/review.
• Rejects governanceVoter == dso, allows bootstrap/self-voting, and adds consuming rotate/clear lifecycle choices.
• Covers create, rotate, clear, unauthorized access, DSO-party rejection, and no-op rotation in Daml tests.

Design IDs: GV-001, GV-003, 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Superseded by consolidated draft PR #6 for single-thread review.