Add a Canton sandbox runbook and seed script #41

User story / Problem statement

As a contributor, I want one command to bring up a local Canton sandbox with the DAR uploaded and an instrument already seeded, so that I can exercise the token and the registry service without hand-assembling parties, a registry, and an instrument config every time.

Expected outcome

A script starts a sandbox with the JSON Ledger API enabled and uploads the production DAR. A seed script allocates the operator and admin parties, creates the registry, and drives propose-accept to produce one instrument, printing the contract ids and party ids the service needs for its configuration. A runbook documents the whole flow.

Acceptance criteria

• A sandbox script builds the production DAR and starts a participant with the JSON Ledger API enabled
• A seed script allocates parties and creates the registry and one instrument through propose-accept
• The seed script prints every value needed to configure the registry service
• A runbook documents prerequisites, the commands, and how to tear down
• Running the two scripts on a clean machine produces a working environment

Technical notes

The seed script drives the JSON Ledger API directly rather than Daml Script, so that it exercises the same API surface the service uses and doubles as an early check on the request envelope.

Additional context

Part of #23. Prerequisite for the end-to-end transfer test and the CLI conformance check.