fix: add paginated device retrieval to prevent DoS on unbounded array return

[pradhyum6144]

Summary

• Adds getDevicesPaginated(uint256 _offset, uint256 _limit) to DeviceRegistry.sol as a gas-safe alternative to getAllDevices()
• Returns a bounded slice of the registered devices array plus total count for client-side paging
• Keeps getAllDevices() for backward compatibility
• 12 Foundry tests covering pagination logic, edge cases, and fuzz

The problem

getAllDevices() returns the entire registeredDevices array in one call. As devices scale, this hits the block gas limit and causes out-of-gas reverts (Issue #14).

PR #18 (LSUDOKO) addresses this at the Node.js server layer. This PR fixes it at the contract level, where the root cause lives.

What changed

DeviceRegistry.sol

New function:

• getDevicesPaginated(_offset, _limit) -> (address[] page, uint256 total)
• Returns at most _limit addresses starting from _offset
• Returns total so callers can compute page count
• Handles all edge cases gracefully (offset beyond length, zero limit, etc.) - never reverts

DeviceRegistryPagination.t.sol - 12 tests

• testPaginatedFirstPage - first 3 of 10
• testPaginatedMiddlePage - offset 3, limit 3
• testPaginatedLastPageTruncated - offset 8, limit 5, only 2 returned
• testPaginatedExactFit - fetch all 10
• testPaginatedLimitLargerThanTotal - caps at array length
• testPaginatedOffsetBeyondLength - returns empty
• testPaginatedOffsetExactlyAtLength - returns empty
• testPaginatedZeroLimit - returns empty
• testPaginatedEmptyRegistry - empty registry
• testPaginatedSingleItem - limit 1
• testFuzz_paginatedNeverReverts - any offset/limit never reverts
• testFuzz_paginatedCoversAll - walking pages covers all devices

All 12 tests pass. Fuzz: 256 runs per test.

Test plan

• forge test - all 12 pagination tests pass
• Fuzz tests confirm no revert for any input combination
• Existing tests unaffected

Addresses #14