Fix variant hints in path errors and IsomorphicSpanningTree complexity

[GiggleLiu]

Summary

• Add variant hint to "no reduction path" error messages in pred path and pred reduce, so users see available variants when a path lookup fails
• Fix IsomorphicSpanningTree<SimpleGraph> worst-case complexity from num_vertices^num_vertices to 2^num_vertices

Fixes #716, fixes #875

Test plan

• make check passes (fmt + clippy + test)
• pred path with nonexistent path shows variant hints
• pred reduce with nonexistent path shows variant hints

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.97%. Comparing base (3024163) to head (110bc79).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1042   +/-   ##
=======================================
  Coverage   97.97%   97.97%           
=======================================
  Files         979      979           
  Lines      100912   100912           
=======================================
+ Hits        98864    98865    +1     
+ Misses       2048     2047    -1     

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