⚡ Bolt: [performance improvement] Replace getEdges() with memory-efficient forEachEdge()

[saurabhsharma2u]

💡 What: Implemented a new forEachEdge method on the Graph class that iterates directly over the internal edges Map and decodes the composite key inline. Replaced all performance-critical usages of graph.getEdges() with graph.forEachEdge(...) in hits.ts, pagerank.ts, and metrics.ts.

🎯 Why: Calling graph.getEdges() inside these hot loops caused massive array and object allocations (creating an array of {source, target, weight} objects for every edge in the graph), leading to significant garbage collection overhead, especially for massive site graphs.

📊 Impact: Reduces heap memory usage substantially during PageRank and HITS calculations, making the application measurably faster and more efficient on large datasets by avoiding full array allocations.

🔬 Measurement: Run pnpm --filter core test to verify functional correctness and see the benchmark outputs from pagerank_perf_large.test.ts and hits_perf_large.test.ts.

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PR created automatically by Jules for task 12845013787333598539 started by @saurabhsharma2u

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