fix: bolt optimization for full graph exports peak memory overhead

[n24q02m]

💡 What: Optimized full graph exports (export_graphml, export_jsonld, export_dot, export_cypher, and export_crg) to stream directly from sqlite3.Cursor rows via new GraphStore.iter_raw_nodes() and GraphStore.iter_raw_edges() methods, instead of loading the entire graph into Python GraphNode/GraphEdge objects.

🎯 Why: Previously, generating exports required materializing tens of thousands of dataclass instances in memory, leading to massive peak memory spikes and CPU overhead during object construction, which is unnecessary since export formats only require scalar row values.

📊 Impact: Significantly reduces peak memory utilization and CPU time during full graph exports (O(1) python object overhead instead of O(N)).

🔬 Measurement: Memory profiling a call to export_crg against a large repository will show substantially lower peak memory overhead as GraphNode and GraphEdge object instantiations are completely eliminated during the export loop.

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PR created automatically by Jules for task 10172509774891061715 started by @n24q02m

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