Rename subgraph ProofSet/Root terminology to DataSet/Piece

[silent-cipher]

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