Add bounded knapsack-augmented cumulative overload checking

[zayenz]

Advanced cumulative propagation can miss overloads when integral task demands cannot fill the apparent spare capacity. This adds the bounded knapsack-augmented overload check (KAOC) from Cloutier and Quimper, CP 2026.

The checker runs after ordinary overload checking and before edge finding under advanced propagation. It admits 4–64 currently mandatory supported tasks, capacities 2–127, and positive demands with at least one non-unit demand. Reachability uses two machine words and transient Region scratch. Optional propagators apply the task-count gate and cost classification to their mandatory subset. KAOC detects conflicts without pruning bounds.

[zayenz]

• The knapsack check rejected feasible fixed-end tasks with variable processing times. It now requires fixed duration before applying the energy check. Endpoint differences are computed in a wide type to avoid overflow. Added a regression with a feasible duration-one schedule.
• Renamed the subset-demand helper to max_reachable and documented the compulsory profile, prefix state, and energy backlog.
• Documented the task, demand, and capacity limits, including which optional tasks participate.

[zayenz]

No actionable findings in the bounded knapsack overload checker. The fixed-duration guard and wide endpoint arithmetic are sound. All 14 CI checks pass at 7bd8aa8cb7; default CI includes cumulative semantic cases but omits the focused knapsack regression.