Wire NNUE eval into real search path; fix critical perf regression

Load-and-call infra: export_nnue_weights.py-produced weight files can
now be loaded at runtime (nnueeval.c's NnueLoadWeights, "nnue load"
command, --nnueweights CLI flag for scripted match-play), and the FC
head (NnueForward) runs the real forward pass against the live
accumulator.

Real integration: Eval() now short-circuits to NnueForward when
g_fUseNnueEval && g_fNnueWeightsLoaded, so ./typhoon can actually play
on the NNUE eval -- ./eval_tune/test_vs_head.sh works unmodified to
compare it against the old HCE. ReInitializeSearcherContext (root.c)
gained a defensive accumulator recompute hook covering setboard's
downstream effect on the next go/ponder; it only fires when both
kings are validly placed, since testrecogn.c's synthetic positions
don't always have one at the moment this is called.

Critical fix: the NNUE accumulator maintenance added to move.c's 8
primitives (and MakeMove/UnmakeMove's king-move backup/restore) was
running unconditionally, on the mistaken assumption this was cheap.
It cost ~50% of engine throughput even with NNUE off (673k nps
measured vs the expected 1.4-1.5M baseline). Gated all of it behind
g_fUseNnueEval; classic HCE is back to ~1.52M nps, NNUE eval runs at
a fair ~409k nps (expected for an unquantized float32 forward pass).

This gating also exposed that testnnue.c's TestNnueAccumulator never
set g_fUseNnueEval, so its own direct NnueAccumRecompute calls raced
ahead of now-gated-off incremental maintenance; fixed by having the
test set/restore the flag around its run.

board.c: added en passant legality check to IsBoardLegal (rank vs
side-to-move, origin-square occupancy) -- a real, previously-latent
bug surfaced while building the NNUE test fixtures, unrelated to
NNUE itself.

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

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