Plumb fThreat and attack snapshots into SPLIT_INFO (prep for step 4)

UNIFY_SEARCH.md step 3: add fThreat and the two attack-snapshot
bitboards (bbMinorAttacksSnapshot/bbRookAttacksSnapshot) to SPLIT_INFO
(chess.h), populated by StartParallelSearch (split.c) from values the
originating thread already has at split-launch time, passed through
from search.c's call site.

Plumbing only -- nothing reads these fields yet (HelpSearch() still
runs its own inline copy, untouched). That's step 4: unify
HelpSearch() onto SearchOneMove, at which point it can pass these
real values instead of the zeroed snapshots and missing fThreat its
current hand-rolled gate has.

Sequenced before step 4 deliberately (not combined or left for
after): landing the plumbing first means there's no window where
HelpSearch() calls the real CanWeReduceOrPruneMove()/ShouldLatePrune()
gate with stale/zeroed snapshots -- see UNIFY_SEARCH.md's note on why
that matters for FH_TABLE_COLLECTION's sampling.

Verified: precommit_check.sh passes. Bit-identical node counts at
sd10/--cpus 1 across all three curated suites, zero change (expected
-- single-threaded path never reads these new fields). cpus 4 smoke
run across all three suites: clean exits, no crashes.

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

0 files changed, 0 insertions, 0 deletions