Recursive RINS

[nguidotti]

This PR implements the recursive RINS heuristics. More specifically, it applies a dive-like procedure to create a RINS sub-MIP, presolved it with Papilo and then passes to the cut-and-branch procedure. It also launch a CPU FJ worker over the sub-MIP that runs in parallel to cut-and-branch.

Since the current implementation, does not have an unified problem representation, this PR also adds:

• A routine for converting a lp_problem_t (standard form) to user_problem_t (range form).
• Allow Papilo to be applied to an user_problem_t instead of optimization_problem_t.
• Adds an upper bound callback for retrieving the upper bound of the main solver (since it may change if another worker find a better incumbent).

Additionally, the number of worker per diving heuristic is no longer tied to the total number of workers, which allows the solver to use all diving heuristics in low thread counts (we rotate between the different heuristics for each diving worker launch).

Results

MIPLIB2017, 10min, GH200

================================================================================
main-2026-07-03 (1) vs recursive-submip (2)
================================================================================
------------------------------------------------------------------------------------------------------------------------------
| | Run 1 | Run 2 | Abs. Diff. | Rel. Diff. (%) |
------------------------------------------------------------------------------------------------------------------------------
| Imported 240 240 +0 --- |
| Feasible 227 228 +1 --- |
| Optimal 86 91 +5 --- |
| Solutions with <0.1% primal gap 138 151 +13 --- |
| Nodes explored (mean) 1.326e+07 1.296e+07 -3.057e+05 -2.31 |
| Nodes explored (shifted geomean) 1.262e+04 1.239e+04 -230 -1.82 |
| Relative MIP gap (mean) 0.2777 0.2726 -0.00512 -1.84 |
| Relative MIP gap (shifted geomean) 0.09089 0.08901 -0.001883 -2.07 |
| Solve time (mean) 422.6 435.4 +12.79 +3.03 |
| Solve time (shifted geomean) 196.9 226.6 +29.66 +15.1 |
| Primal gap (mean) 9.969 9.29 -0.679 -6.81 |
| Primal gap (shifted geomean) 0.4584 0.3491 -0.1093 -23.8 |
| Primal integral (mean) 25.02 22.97 -2.051 -8.2 |
| Primal integral (shifted geomean) 2.849 2.421 -0.4275 -15 |
------------------------------------------------------------------------------------------------------------------------------
----------------------------------------------------------------------
| Name | status 1 | status 2 |
----------------------------------------------------------------------
| neos-1171737 feasible optimal |
| radiationm40-10-02 feasible optimal |
| rail01 timeout feasible |
| ran14x18-disj-8 feasible optimal |
| triptim1 feasible optimal |
| unitcal_7 feasible optimal |
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[chris-maes]

This comment should move to the implementation, so that the reader can see the math next to the code. And doesn't have to look for it in another file.

[chris-maes]

Thanks for the PR @nguidotti ! Congratulations on the performance improvements.

Please address the minor comment before merging. Thanks!

[nguidotti]

/merge