feat: add CodSpeed benchmark tests for graph operations

[hneiva]

Add pytest-codspeed benchmarks testing core graph operations (transitive closure, topological sort, link dicts, for_each_task, JSON round-trip, and transform sequences) across 4 graph geometries with 1000 tasks each: linear chain, fan-out/fan-in, binary tree, and diamond layers.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.28%. Comparing base (e479436) to head (5c694db).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #922      +/-   ##
==========================================
+ Coverage   77.07%   77.28%   +0.20%     
==========================================
  Files         130      131       +1     
  Lines       11824    11933     +109     
  Branches     1451     1459       +8     
==========================================
+ Hits         9113     9222     +109     
  Misses       2330     2330              
  Partials      381      381              

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