[E1-2] Governance action `DsoRules_RegisterSynchronizer`

[salindne]

New SRARC_RegisterSynchronizer variant + choice + result + dispatch arm (mirrors DsoRules_CreateUnallocatedUnclaimedActivityRecord). A vote creates the splice-amulet registry contract.

Key files: daml/splice-dso-governance/daml/Splice/DsoRules.daml:122 (variant), :1699 (choice), :342 (result)

Acceptance criteria:

• Vote-to-create path exercised by TestRegisterSynchronizer

Depends on: E1-1

Epic: #4

Source: docs/planning/extending-mainnet-work-plan.md