Add alpha-side static null-move skip gate and Search() perf hoists

Reuses the joint FH% table on the iEval <= iAlpha side to skip the
null-move probe attempt when it's predicted unlikely to succeed
(EV-neutral: the real move loop still runs regardless). Also hoists
several node-constant computations (ImproveBucket, MP split-eligibility
checks) out of the per-move loop in Search(), and adds GNUmakefile
overrides for sweeping NULLMOVE_SKIP_FH_CEILING_THOUSANDTHS and
SKIP_SELECT_AFTER_N_MOVES from the command line.

Self-play vs. head is trending positive overnight (1020 games,
llr=+1.36, score=0.527) but not yet conclusive -- committing now to
checkpoint the change while the match continues.

Co-Authored-By: Claude Sonnet 5 <[email protected]>

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