Fix board corruption when an undo interrupts the practice-mode evaluation

[oers]

This fixes the recurring WthorReplayTest failure "WThor game 155 undo of ply 28 (g8) did not clear that square". It has shown up on #96, #106, #112 and #114.

Cause

In practice mode, extended_compute_move (zebra/game.c) evaluates every legal move. For each candidate it makes the move, searches, and takes the move back. If the opponent has to pass after the candidate, it runs a second search for the same side.

An undo or redo interrupts the evaluation by setting force_return. The check after the first search took the move back before leaving the loop, but the check after the second search did not. So an undo that arrived during the second search left the candidate move on the board, and the position shown after the undo was wrong.

Game 155 after ply 28 is exactly that situation: White has to pass after most of Black's moves. That is why the failure always appeared there, and only when the timing hit the second search.

Fix

unmake_move is now also called before leaving on force_return after the second search, the same way as after the first search.

Test

New host test HostJniSmokeTest#undoDuringAPracticeEvaluationLeavesTheBoardIntact:

• It loads game 155 up to ply 28 with practice mode on.
• It undoes that ply 60 times, at varying points in the evaluation, and redoes it after each undo.
• After each undo it compares the board with the known position after ply 27.

Results on the host (x86_64):

• Without the fix, the test fails on the first attempt: a4: expected EMPTY, actual BLACK, …. The candidate a4 was left on the board.
• With the fix, all 60 attempts pass, in three separate runs.
• All 13 host JNI tests pass, including allWthorGamesUndoRedoRoundTripCleanly.

🤖 Generated with Claude Code

https://claude.ai/code/session_01V5G4cWSLBkiNNK4KC3zvXM

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