Fix negative weights in vertex cover to AND/OR graph reduction

[isPANN]

Summary

• Include every negative-weight vertex in the recovered cover, even if isolated or redundant.
• Reduce only edges not already covered by these forced vertices, using nonnegative residual weights.
• Report target size formulas as upper bounds and document the residual optimum and constant objective offset.
• Preserve the existing witness extraction API. Three files, based directly on main.

Verification

• Regression test fails on main because recovered covers are not optimal.
• Exhaustive source/target checks cover isolated negative vertices, adjacent negative vertices, mixed signs, and self-loops.
• Full workspace all-feature tests: 6,479 passed; 2 existing ignored.
• Focused reduction tests: 5 passed; changed-line coverage 100% across 29 lines.
• Formatting, diff whitespace checks, workspace all-target all-feature Clippy with warnings denied, and make paper passed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.93%. Comparing base (7dd5fcd) to head (b8a5765).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1160   +/-   ##
=======================================
  Coverage   95.93%   95.93%           
=======================================
  Files        1074     1074           
  Lines      132106   132130   +24     
=======================================
+ Hits       126730   126754   +24     
  Misses       5376     5376           

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[isPANN]

Consolidated into #1155 for one review of combinatorial reduction correctness fixes. No changes have been merged to main.