⚡ Bolt: Avoid libm::sqrt in ManifoldPoint::is_neighbor

[teerthsharma]

💡 What: Replaced the libm::sqrt-based exact distance calculation with an inline squared distance loop in ManifoldPoint::is_neighbor. Implemented early exit when the squared distance sum exceeds the squared epsilon (eps_sq) and added robust handling for NaN/negative inputs using ! logic.

🎯 Why: SparseAttentionGraph::add_point calls is_neighbor inside an $O(N)$ loop for every point added, making it a critical hot path for spatial graph construction. The sqrt operation is expensive and unnecessary for simple threshold comparisons.

📊 Impact: Significantly speeds up the sparse attention mechanism by completely avoiding square root operations and short-circuiting dimension evaluation when points are provably outside the radius.

🔬 Measurement: Run cargo test -p aether-core manifold to ensure correctness. The performance gain can be seen in microbenchmarks or any workloads generating continuous spatial structures.

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PR created automatically by Jules for task 6886637532171051806 started by @teerthsharma

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