Delegator Visibility & Earnings Transparency

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## Goal

Delegators can view their per-round reward history and export earnings data from the Explorer.

## Context

Delegators currently have no visibility into how their stake grows round-over-round. The Explorer doesn't show per-round reward breakdowns, historical stake, or provide any export functionality for tax reporting. This requires subgraph-level changes (cumulative factors and delegator shares) to make the data queryable without contract calls, followed by Explorer UI work to surface it.

Upstream subgraph work: livepeer/subgraph#217

## Scope

**Included:** Subgraph cumulative factor and delegator shares support, per-round reward history in Explorer, CSV export of delegator/orchestrator history.

**Not included:** Advanced analytics dashboards, yield calculators, or third-party integrations.

## Success Criteria

- Delegators can see LPT (and USD value) earned per round in the Explorer
- Users can export orchestrator/delegator history as CSV for a given time range
- All data is served from the subgraph without contract calls

Start by reviewing the upstream livepeer/subgraph#217 work for cumulative factors and delegator shares, then inspect how the Explorer currently presents delegator and orchestrator history. The work is done when per-round LPT and USD rewards are visible, history can be exported as CSV for a chosen range, and the data comes from the subgraph without contract calls.

Written by the indexing model from the issue text.