[P2-E1] On-ledger registration & governance

The DSO maintains an on-ledger registry mapping a dedicated synchronizer id to its Sync
Operator (SO) party. Registration is the authorization anchor for everything downstream:
the buy gate, fee binding, discount, reconciliation, and reporting all key off it. The
registry must be publicly readable (FR-1) and governance must hold an offboard lever
(FR-2). Splice surface: new template in `splice-amulet` (or `splice-dso-governance`),
vote actions in `DsoRules`.

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Milestone: Phase 2 · Plan: [P2-E1-registration.md](https://github.com/ChainSafe/canton-extending-mainnet/blob/main/docs/planning/phase-2-epics/P2-E1-registration.md) · Leaves filed as sub-issues.

Start with docs/planning/phase-2-epics/P2-E1-registration.md to understand the registration scope and requirements. Then inspect the proposed template location in splice-amulet or splice-dso-governance and the vote actions in DsoRules. Done means the registry is publicly readable and governance provides the specified offboard lever.

Written by the indexing model from the issue text.