Simplify internal graph checks and Steiner witness extraction

[isPANN]

Summary

• Replace the private articulation-point DFS with petgraph connectivity and articulation-point checks.
• Replace private union-find implementations in forest partitioning and PERT network evaluation with the existing petgraph dependency.
• Extract prize-collecting Steiner forest witnesses in one edge scan, without repeatedly cloning the target edge list.
• Preserve public APIs and feasibility semantics, including empty graphs, disconnected graphs, loops, repeated edges, and zero-prize connector vertices.
• Six files, 57 added lines and 134 removed lines; based directly on main.

Verification

• Full workspace all-feature unit and integration tests: 6,479 passed; 2 existing ignored.
• Library coverage run includes the graph checks and Steiner closed-loop tests.
• Changed-line coverage: 100% of 47 measured lines.
• Workspace all-target all-feature Clippy with warnings denied, formatting, and diff whitespace checks passed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.92%. Comparing base (7dd5fcd) to head (4c2b8eb).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1158      +/-   ##
==========================================
- Coverage   95.93%   95.92%   -0.01%     
==========================================
  Files        1074     1074              
  Lines      132106   132060      -46     
==========================================
- Hits       126730   126684      -46     
  Misses       5376     5376              

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[GiggleLiu]

Final review: petgraph replacements preserve the K2 convention and cycle detection; merged extractor loop is equivalent. make check passes locally on the merged state. Approving as-is.