Replace offset-based active piece pagination with `getActivePiecesByCursor`

[BravoNatalie]

Description

synapse-core currently exposes and uses getActivePieces(setId, offset, limit) for active piece pagination, but this path becomes impractical on large data sets. In PDPVerifier, getActivePieces has O(offset) gas complexity because it always iterates from piece ID 0. For deep pages, read calls can fail with out-of-gas errors even though the data set is live and the call is otherwise valid.

The contract already includes getActivePiecesByCursor(setId, startPieceId, limit), which is designed for O(limit) pagination and works correctly for the same data sets where offset-based pagination fails. However, this function is not currently exported by synapse-core, which forces downstream consumers onto the more expensive path.

Proposed solution

Add a getActivePiecesByCursor export to @filoz/synapse-core/pdp-verifier and update internal or downstream pagination flows to prefer it over getActivePieces.

Done criteria

• synapse-core exports getActivePiecesByCursor from @filoz/synapse-core/pdp-verifier
• The helper includes a typed call builder and parsed return shape consistent with existing read helpers
• Offset-based pagination usage is replaced with cursor-based pagination in the relevant active-piece listing
  flow(s)
• Tests cover first-page and subsequent-page cursor pagination behavior
• Docs or examples note that getActivePieces is O(offset) and getActivePiecesByCursor should be preferred for
  large data sets

[BigLep]

@SgtPooki and @BravoNatalie : I see this was moved to in progress. Is it actually being worked on? I'm moving it back to "Todo" but let me know if that isn't the case.