Three correctness fixes to the VCN search implementation covering stand-pat semantics in VC4 mode, VC5 quick-check ordering, and safe use of PASS as a TT best move.

VC4 stand-pat disabled in vcfsearch()

In VC4 mode the attacker must find a VCF sequence — returning static eval as "good enough" is wrong. vcfsearch() now initialises bestValue = mated_in(ss->ply + 2) when vcnAllowB4, bypassing stand-pat and delta-pruning entirely:

• No VCF moves found → returns mated_in(ss->ply + 2) (correct)
• VCF moves found → loop updates bestValue normally
• Single unified if (PvNode && bestValue > alpha) alpha = bestValue; replaces the two duplicated alpha-update branches

VC5 quick check reordered

Vcn::vc5AttackerLoses() is now evaluated before the board-filled/max-ply draw check in both search() and vcfsearch(). A VC5 attacker with no A_FIVE should always return mated_in(ss->ply), never a draw score, even when the board is full.

PASS as TT best move

The hashtable already encodes Pos::PASS safely (move + 1 offset, see hashtable.cpp), so the previous filter bestMove == Pos::PASS ? Pos::NONE : bestMove in TT.store is reverted. Instead, the three call sites that reach board.cell(ttMove) — which asserts pos >= 0 and crashes for Pos::PASS — are individually guarded:

updateBestmoveStats retains its existing bestMove != Pos::PASS guard unchanged.

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