feat: add CustomizedSolver with structure-exploiting backends

[GiggleLiu]

Summary

• Add CustomizedSolver — a new solver backend for exact witness recovery using structure-exploiting search, exposed via pred solve --solver customized
• Implement three backend families:
  • FD subset search: shared branch-and-bound with closure pruning for MinimumCardinalityKey, AdditionalKey, PrimeAttributeName, BoyceCoddNormalFormViolation
  • Bounded cycle hitting: enumerates short cycles then finds minimum hitting set for PartialFeedbackEdgeSet
  • Tree arrangement backtracking: interleaves tree construction and permutation mapping for RootedTreeArrangement
• Wire into CLI dispatch (--solver customized), with helpful error messages for unsupported problems

Test plan

• 19 unit tests: parity checks against BruteForce, exact witness validation, edge cases (no solution, tight bounds)
• 2 CLI integration tests: unsupported problem hint, end-to-end solve
• All 3360 existing tests pass (zero regressions)
• Pre-existing clippy warnings in ilp_helpers.rs are unrelated to this PR

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 96.23477% with 34 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.82%. Comparing base (bc974d1) to head (abe7fcb).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/solvers/customized/fd_subset_search.rs	91.39%	13 Missing ⚠️
src/unit_tests/solvers/customized/solver.rs	96.15%	10 Missing ⚠️
src/solvers/customized/rooted_tree_arrangement.rs	97.19%	5 Missing ⚠️
...rc/solvers/customized/partial_feedback_edge_set.rs	97.77%	3 Missing ⚠️
src/solvers/customized/solver.rs	98.30%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #766      +/-   ##
==========================================
- Coverage   97.84%   97.82%   -0.03%     
==========================================
  Files         583      588       +5     
  Lines       65521    66422     +901     
==========================================
+ Hits        64112    64976     +864     
- Misses       1409     1446      +37     

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