Add residual top-up extraction to PPR forward push

[mkolodner-sc]

Summary

Adds an opt-in residual top-up extraction API to PPRForwardPush.

The existing extractTopK behavior is unchanged. The new API returns the normal top-k finalized PPR nodes first, then appends additional candidates from residual mass that was discovered during forward push but did not necessarily cross the requeue threshold. This gives downstream samplers a way to fill more sequence slots without issuing extra neighbor fetches.

Changes

• Adds extractTopKWithResidualTopUp(maxPprNodes, maxResidualNodes) in C++.
• Exposes the new method through pybind and the Python stub.
• Scores residual top-up candidates on the same scale as finalized candidates via ppr_score + residual.
• Improves neighbor cache behavior so previously fetched (node, edge type) neighbor lists are reused within a PPR state.
• Adds C++ tests for neighbor cache reuse and residual top-up extraction.

[mkolodner-sc]

Closing in favor of #702, which uses the renamed mkolodner/ppr-residual-topup branch.