perf: batch DynamoDB degree queries

[j6k4m8]

Summary

• compute DynamoDB bulk degree results from one paginated edge scan
• preserve indexed queries for single-node degree calls
• support directed, undirected, filtered, and all-node bulk results
• preserve caller key types and include requested zero-degree nodes

[codspeed-hq]

Merging this PR will not alter performance

✅ 17 untouched benchmarks

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_{Comparing perf/dynamodb-bulk-degrees-83 (5f954ab) with master (a516303)}