Implement the Blockfrost governance data provider

## Outcome

The Governance API can use Blockfrost for the supported governance-data capability set.

## Scope

- Confirm the Blockfrost capability matrix
- Implement canonical mappings and provider errors
- Handle authentication, rate limits, retries, and pagination
- Pass applicable provider contract tests

## Acceptance criteria

- [ ] Scope is decomposed into independently deliverable child issues.
- [ ] Architecture and API decisions are linked from this issue.
- [ ] Test, documentation, observability, rollout, and failure behavior are defined where applicable.
- [ ] Epic exit criteria are verified before closure.

## Planning note

This is a 2026 GovTool revamp work item. Its child features and tasks should be added as the scope is refined.

Start by decomposing the Blockfrost capability matrix and identifying the applicable provider contract tests. Link the architecture and API decisions, then define authentication, rate-limit, retry, pagination, testing, documentation, observability, rollout, and failure behavior. Done means the child issues and epic exit criteria are explicitly defined and verified.

Written by the indexing model from the issue text.