execute

Executes the fixed‑K swap mutation on candidate. If the candidate is not a BestSubsetSolution, it is lifted to one (bitset and objectives are copied).

Steps:

1. Early exit if mutation does not trigger or there are no bits.

2. Snapshot current mask into a primitive array.

3. Collect indices with 1 and with 0.

4. Perform:

• Swap if both pools have elements (K preserved).

• Repair if the mask is all‑zeros or all‑ones (K set to 1 or N‑1).

5. Record swap/repair metadata for delta‑evaluation.

Parameters