Rename subgraph ProofSet/Root terminology to DataSet/Piece

## Context

The PDP terminology has moved from:

- `ProofSet` -> `DataSet`
- `Root` -> `Piece`

The subgraph is currently mixed: the primary entity is already `DataSet`, but much of the GraphQL schema, derived fields, metrics, helper names, mappings, comments, and the doc still expose or use `proofSet` and `root` terminology.

## Proposed work

Update the subgraph naming so public GraphQL API and internal code consistently use DataSet/Piece terminology.

## Acceptance criteria
- GraphQL schema exposes DataSet/Piece terminology consistently.
- Subgraph mappings compile agains the renamed generated schema.
- Templates include the renamed entities.
- Tests are update from `Roots`/`proofSet` assertions to `Piece`/`dataSet`.
- No subgraph code uses `ProofSet` or `Root` terminology except where referring to historical migrations or external contract/event names.
- Documentation is updated or tracked in a separate docs issue.
- Make sure downstream consumers (e.g. dealbot) are updated

Start with the GraphQL schema and generated schema used by the subgraph mappings, then inspect templates and tests for Roots, proofSet, and related assertions. Done means the schema, mappings, templates, and tests consistently use DataSet/Piece terminology, historical or external names are preserved only where required, documentation is updated or tracked separately, and downstream consumers such as dealbot are accounted for.

Written by the indexing model from the issue text.