Honor forced pairs in Murty subproblems

[FlorianPfaff]

Summary

• make Murty child subproblems actually enforce their forced prefix row/column pairs
• reject contradictory forced/forbidden pairs before solving a child subproblem
• add a regression covering duplicate k-best assignment output from an ignored forced prefix

Bug fixed

murty_k_best_assignments() records a forced prefix when branching, but _solve_subproblem() only checked for duplicate forced rows/columns and then restored the forced cell. It did not mask the rest of the forced row and column, so the linear assignment solver could ignore the forced prefix. On tied/near-tied partial-assignment cases this could repeat an earlier assignment and skip a valid next-best alternative.

Testing

• Added tests/utils/test_assignment_murty_forced_prefix.py.
• Inspected main..fix-murty-forced-prefix: 2 files changed, +40/-1.
• Full test suite not run locally; this environment cannot resolve github.com for cloning/installing the repo.

[github-actions]

✅MegaLinter analysis: Success

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✅ PYTHON	isort	1201	65	0	0	2.59s
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✅ REPOSITORY	gitleaks	yes		no	no	9.18s
✅ REPOSITORY	git_diff	yes		no	no	0.29s
✅ REPOSITORY	secretlint	yes		no	no	30.13s
✅ REPOSITORY	syft	yes		no	no	3.78s
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✅ REPOSITORY	trufflehog	yes		no	no	14.36s
✅ YAML	prettier	11	0	0	0	0.55s
✅ YAML	v8r	11		0	0	8.35s
✅ YAML	yamllint	11		0	0	0.42s

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