refactor(core): fetch node/edge data lazily

[jnooree]

Checklist

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Linked Issues

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• Closes Fetch node/edge data lazily #216.

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[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (d7534c3) 91.09% compared to head (c72a2b3) 91.10%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #217   +/-   ##
=======================================
  Coverage   91.09%   91.10%           
=======================================
  Files          17       17           
  Lines        4078     4080    +2     
=======================================
+ Hits         3715     3717    +2     
  Misses        363      363           

Files	Coverage Δ
include/nuri/core/graph.h	97.74% <96.15%> (+<0.01%)	⬆️