[WIP] Add pushout of sheaves over a span of graph homomorphisms

[Copilot]

• Understand prerequisites: pushforward_sheaf, graph_pushout, pushout_sheaf (all merged)
• Create src/network_sheaves/PushoutSheaves.jl with:
  • _fill_missing_edges! helper
  • _induced_sheaf_morphism helper
  • _build_canonical_morph helper
  • pushout_sheaf_over_span main function
• Update src/network_sheaves/NetworkSheaves.jl (include + reexport)
• Create test/network_sheaves/PushoutSheaves.jl
• Update test/runtests.jl
• Run tests

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.90%. Comparing base (92b2276) to head (201991e).
⚠️ Report is 38 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #35      +/-   ##
==========================================
- Coverage   84.06%   82.90%   -1.17%     
==========================================
  Files          13       13              
  Lines        1086     1234     +148     
==========================================
+ Hits          913     1023     +110     
- Misses        173      211      +38     

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[Copilot]

Pull request overview

This WIP PR currently only modifies the test environment by adding CellularSheaves as an explicit dependency in test/Project.toml.

Changes:

• Add CellularSheaves to test/Project.toml [deps].

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