feat(round): implement paginated bidder reads end-to-end

[NnamdiCyber]

Problem

The existing get_bidders view returns the entire bidder index as one array. For rounds with hundreds or thousands of bidders, this forces the keeper to load all addresses into memory at once and risks hitting Soroban RPC response size limits.

Solution

Add get_bidders_page — a zero-based cursor/limit paginated view — across all layers, keeping the original get_bidders fully intact for backward compatibility.

Contract (contracts/round/)

BiddersPage {
  data:        Vec<Address>,   // page of bidder addresses
  next_cursor: u32,            // 0 = no more pages
  total:       u32,            // total bidders in the round
}
- MAX_PAGE_SIZE = 100, InvalidLimit = 39 error code
- cursor is zero-based; next_cursor = 0 signals end-of-list (avoids Soroban XDR Option<u32> complexity)
- 12 new pagination tests (empty, partial, exact, final, multi-page, invalid limits, cursor at/beyond total, order preservation)
- Existing get_bidders unchanged and fully compatible
Bindings (@sub-rosa/round-bindings)
- BiddersPage TypeScript interface
- get_bidders_page method on Client
- Hand-crafted XDR spec entries for struct and function
- InvalidLimit error code mapping
SDK (@sub-rosa/sdk)
- getBiddersPage(roundId, cursor, limit) — fetch one page
- async *bidders(roundId) — lazy async generator that pages through all bidders
- 3 encoding tests verifying get_bidders_page argument encoding
Keeper
- keepRound uses sdk.bidders() async generator instead of bulk sdk.getBidders()
- Same per-bidder reveal logic, addresses are now fetched page-by-page
Backward Compatibility
All existing callers using get_bidders continue to work without changes. The old function remains exported and functionally identical.

Closes #7

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