[E1-1] `RegisteredSynchronizer` template + public fetch

[salindne]

DSO-signed syncId -> operator party registry, operator as observer, with a public RegisteredSynchronizer_Fetch (mirrors OpenMiningRound_Fetch) for explicit-disclosure reads. Add a lifecycle-state field for offboard/revoke.

Key files: daml/splice-amulet/daml/Splice/DecentralizedSynchronizer.daml:120

Acceptance criteria:

• Template + fetch compile; lifecycle-state field present
• Not conflated with requiredSynchronizers or DSO synchronizers config

Epic: #4

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