Expose voting metrics, power, and rationales consistently

## Outcome

Voting activity, live metrics, voting power, and rationales use consistent semantics across DReps, Governance Actions, and providers.

## Scope

- Define calculation rules and provenance for vote metrics
- Define voting-power snapshots and historical semantics
- Normalize vote rationale retrieval and validation state
- Add cross-provider fixtures and 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 voting metrics, voting-power, rationale, and cross-provider fixture scope into independently deliverable child issues. Define the architecture and API decisions, then specify test, documentation, observability, rollout, and failure behavior requirements. Done means the child issues and linked decisions cover the stated epic exit criteria.

Written by the indexing model from the issue text.