feat: add getActivePiecesByCursor for gas-efficient pagination

[Chaitu-Tatipamula]

Summary

Fixes #243 - Adds getActivePiecesByCursor function that uses cursor-based pagination with O(limit) gas complexity, avoiding the gas exhaustion issue with getActivePieces at high offsets.

Changes

• New function: getActivePiecesByCursor(setId, startPieceId, limit) - starts from startPieceId instead of scanning from 0
• Updated docs: Added warning to getActivePieces about O(offset) gas complexity
• Tests: Added 6 new tests covering basic pagination, deleted pieces, edge cases

Usage

// First page
(pieces, ids, hasMore) = getActivePiecesByCursor(setId, 0, 100);

// Next page: use last returned pieceId + 1
(pieces, ids, hasMore) = getActivePiecesByCursor(setId, ids[ids.length-1] + 1, 100);

[rvagg]

Merging, but this isn't going live until we can schedule an upgrade

[rvagg]

Thanks @Chaitu-Tatipamula, good work