Engine direction: keep improving the native engine, or integrate Stockfish?

[testtest126]

The native engine has come a long way — iterative deepening, aspiration windows, transposition table (now persistent, with pondering after #16), null-move pruning, killers/history ordering, quiescence, opening book — with fully deterministic, node-count-tested search. Depth-4 from the start position costs ~3k nodes. But it evaluates with material+PST and will plateau around club strength.

Two roads from here:

A. Keep going native. Next rungs: late-move reductions, static exchange evaluation for capture ordering, futility pruning, a real evaluation (king safety, pawn structure, passed pawns — or a small NNUE). Pros: everything stays testable/deterministic, pure Swift, no licensing friction. Cons: months of work to reach what Stockfish gives for free; strength claims are hard to calibrate.

B. Integrate Stockfish for "Expert+" and review. The ChessEngine protocol was designed for exactly this swap — UI and game logic wouldn't change. Pros: instant world-class analysis, trustworthy review accuracy. Cons: C++ build integration, GPLv3 obligations for distribution (App Store distribution of GPL code needs care), binary size, and the native engine work becomes a hobby lane.

A hybrid seems plausible: native engine stays the opponent at all levels (its beatable, human-ish levels are a feature), Stockfish powers post-game review only. Opinions?