perf: query DynamoDB adjacency indexes

[j6k4m8]

Summary

• add source and target global secondary indexes to newly created edge tables
• replace neighbor and predecessor scans with paginated index queries
• query both indexes for undirected adjacency and deduplicate self-loops
• normalize indexed endpoint values to strings
• cover table schema, pagination, query direction, and scan avoidance with isolated mocks

[codspeed-hq]

Merging this PR will not alter performance

✅ 8 untouched benchmarks

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_{Comparing fix/dynamodb-adjacency-queries-69 (bb6d11a) with master (26b27fa)}