Mod-2 cut separator for zero-half cuts (#1726)

- Add general mod-2 zero-half separation. The separator finds combinations of constraint rows whose odd coefficients cancel modulo 2 while their aggregated right-hand side remains odd. Halving and rounding this aggregate yields a valid cut.
- Use sparse GF(2) elimination to find these row dependencies, then transform, lift, score, and add violated candidates to the cut pool.
- Propagate B&B timeouts to active node solves and improve work unit checks for some cuts.
- Update PaPILO pin for the coefficient-capacity guard fix.



On H100, two runs each:
Main vs mod-2
**Feasible** | 224.0 | 226.5 | +2.5
**Average error gap** | 12.285 | 11.655 | -0.630
**Optimal instances** | 76.5 | 77.5 | +1.0
**Root gap closed average** | 29.7518% | 31.0948% | +1.3429%
**Root gap closed shifted geomean** (+1) | 10.4875% | 11.1626% | +0.6750%
MIP gap unchanged, due to some cut passes taking longer.

Authors:
  - Akif ÇÖRDÜK (https://github.com/akifcorduk)

Approvers:
  - Alice Boucher (https://github.com/aliceb-nv)
  - Nicolas L. Guidotti (https://github.com/nguidotti)
  - Ramakrishna Prabhu (https://github.com/ramakrishnap-nv)

URL: #1726

Author: akifcorduk

cpp/CMakeLists.txt

@@ -277,7 +277,7 @@ FetchContent_Declare(
         # This is the reason we are using the development branch
         # from Oct 12, 2025. Once these changes are merged into the main branch,
         #we can switch to the main branch.
-        GIT_TAG "32b3a87dbf4955d5a2803be74145c389ea31434d"
+        GIT_TAG "55d5edece584885061639ecf3a6eb8a4629be9f2"
         GIT_PROGRESS TRUE
         EXCLUDE_FROM_ALL
         SYSTEM

cpp/src/branch_and_bound/branch_and_bound.cpp

@@ -1565,8 +1565,9 @@ dual_status_t branch_and_bound_t<i_t, f_t>::solve_node_lp(
   } else {
     lp_settings.cut_off = cutoff + settings_.dual_tol;
   }
-  lp_settings.inside_mip      = 2;
-  lp_settings.time_limit      = settings_.time_limit - toc(exploration_stats_.start_time);
+  lp_settings.inside_mip = 2;
+  lp_settings.time_limit = settings_.time_limit - toc(exploration_stats_.start_time);
+  if (lp_settings.time_limit <= 0.0) { return dual_status_t::TIME_LIMIT; }
   lp_settings.scale_columns   = false;
   lp_settings.iteration_limit = iter_limit;
 
@@ -1823,6 +1824,10 @@ void branch_and_bound_t<i_t, f_t>::plunge_with(bfs_worker_t<i_t, f_t>* worker,
     abs_gap     = compute_user_abs_gap(original_lp_, upper_bound, lower_bound);
   }
 
+  if (solver_status_ == mip_status_t::TIME_LIMIT || solver_status_ == mip_status_t::OPTIMAL) {
+    node_concurrent_halt_ = 1;
+  }
+
   // If the solver exits early without consuming the local stack, or converged according to
   // the gap rules while nodes are still pending, put those nodes back into the global queue
   // before returning.
@@ -1840,6 +1845,9 @@ void branch_and_bound_t<i_t, f_t>::plunge_with(bfs_worker_t<i_t, f_t>* worker,
 template <typename i_t, typename f_t>
 void branch_and_bound_t<i_t, f_t>::launch_bfs_worker(bfs_worker_t<i_t, f_t>* worker)
 {
+  // The status may change after the caller checks its search-loop condition.
+  if (solver_status_ != mip_status_t::UNSET) { return; }
+
   bfs_worker_t<i_t, f_t>* idle_worker = bfs_worker_pool_.pop_idle_worker();
   if (!idle_worker) return;
 
@@ -1976,8 +1984,7 @@ void branch_and_bound_t<i_t, f_t>::best_first_search_with(bfs_worker_t<i_t, f_t>
     rel_gap     = user_relative_gap(user_obj, user_lower);
 
     if (abs_gap <= settings_.absolute_mip_gap_tol || rel_gap <= settings_.relative_mip_gap_tol) {
-      node_concurrent_halt_ = 1;
-      solver_status_        = mip_status_t::OPTIMAL;
+      solver_status_ = mip_status_t::OPTIMAL;
       break;
     }
 
@@ -1987,6 +1994,10 @@ void branch_and_bound_t<i_t, f_t>::best_first_search_with(bfs_worker_t<i_t, f_t>
     }
   }
 
+  if (solver_status_ == mip_status_t::TIME_LIMIT || solver_status_ == mip_status_t::OPTIMAL) {
+    node_concurrent_halt_ = 1;
+  }
+
   // If the worker has still nodes in the queue (this can happen if it was stopped due to
   // time limit, small gap or other reason), then do not add back to the pool to avoid
   // constantly trying to start it again
@@ -2044,7 +2055,8 @@ void branch_and_bound_t<i_t, f_t>::dive_with(diving_worker_t<i_t, f_t>* worker,
     }
 
     if (toc(exploration_stats_.start_time) > settings_.time_limit) {
-      solver_status_ = mip_status_t::TIME_LIMIT;
+      node_concurrent_halt_ = 1;
+      solver_status_        = mip_status_t::TIME_LIMIT;
       break;
     }
     if (dive_stats.nodes_explored >= diving_node_limit) { break; }
@@ -2063,7 +2075,8 @@ void branch_and_bound_t<i_t, f_t>::dive_with(diving_worker_t<i_t, f_t>* worker,
     ++dive_stats.nodes_explored;
 
     if (lp_status == dual_status_t::TIME_LIMIT) {
-      solver_status_ = mip_status_t::TIME_LIMIT;
+      node_concurrent_halt_ = 1;
+      solver_status_        = mip_status_t::TIME_LIMIT;
       break;
     }
     if (lp_status == dual_status_t::CONCURRENT_LIMIT) { break; }

cpp/src/cuts/CMakeLists.txt

@@ -6,6 +6,7 @@
 set(CUTS_SRC_FILES
   ${CMAKE_CURRENT_SOURCE_DIR}/cuts.cpp
   ${CMAKE_CURRENT_SOURCE_DIR}/objective_step.cpp
+  ${CMAKE_CURRENT_SOURCE_DIR}/zero_half_mod2.cpp
   )
 
 set(CUOPT_SRC_FILES ${CUOPT_SRC_FILES}