Add fixed-depth minimax to value-head search

[natsu0529]

Summary

• add configurable full-width fixed-depth minimax to valuehead
• preserve depth 1 as the default and let go depth override ValueHeadDepth
• reduce requests to the deepest complete tree that fits ValueHeadMaxNodes
• score checkmates, draws, repetitions, and the fifty-move rule without unnecessary NN evaluation
• run instamove searches asynchronously with stop/abort cancellation and serialized UCI output
• add focused regression tests for minimax, terminal scoring, safeguards, black-to-move handling, and response races

Behavior and safeguards

ValueHeadDepth defaults to 1, so the existing default behavior is preserved. ValueHeadMaxNodes defaults to 10000. If a requested full-width tree exceeds that limit, the search uses the deepest complete depth that fits; if even depth 1 does not fit, it returns a legal fallback move without NN evaluation.

Instamove search now runs on a worker thread. Tree construction, evaluation queuing, and minimax resolution observe cancellation. info and bestmove output are serialized so no search info is emitted after the final move.

Testing

• ninja -C builddir
• meson test -C builddir --print-errorlogs — 9/9 test targets passed
• InstamoveTest — 14/14 cases passed
• clang-format dry run and git diff --check
• UCI smoke test with ValueHeadMaxNodes=300000, go depth 4, and stop after 10 ms

The default depth remains unchanged, so this does not alter default value-head playing behavior

Related to #2176.