[KnightStackSys]

Summary

• Stop regular iterative deepening after a completed root iteration returns a winning mate and valid best move.
• Avoid stopping on intermediate aspiration bounds, losing mates, ordinary cp scores, or missing root moves.
• Preserve UCI go infinite and active pondering until stop/ponderhit.
• Apply the same behavior to MultiPV only after the complete depth's requested PV lines have been emitted.
• Add focused predicate tests and end-to-end four-player Teams king-capture mate tests for regular search and MultiPV.
• Document behavior in docs/mate-stop.md.

Validation

Linux GCC and Windows MSVC CI pending. No changes to scoring, pruning, or move-ordering heuristics.