Make the FH% regression model the default; remove data.c's retired tables

Flips FH_USE_REGRESSION_MODEL from an opt-in safety toggle to the only
behavior: fhmodel.c's quantized logistic regressions are now always
used, with no build-time choice left between them and the old
histogram tables. Removes g_uJointFHThousandths/g_uBetaFHThousandths
and their _NNUE counterparts from data.c (4085 lines), their extern
declarations from chess.h, and the now-dead
_JointTablePredictedFHThousandths/_BetaTablePredictedFHThousandths
lookup functions and their supporting bucket helpers
(HangCpBucket/GapCpBucket/MoveNumBucket/DepthBucket/ImproveBucket/
GapBetaBucket/_ShipDepthBucket/_ShipLateBucket) from searchsup.c, along
with the uRawDepthB/uRawImproveB plumbing that only existed to feed
them (search.c, split.c, SearchOneMove's parameter list).

Also drops the beta-side static-nullmove skip threshold's provisional-
96000 #else branch -- 99000 is now simply the default, confirmed (not
provisional) by docs/REGRESSION.md's trust-audit sweep (+291 Elo over
96000 at 200 games, LOWER95=0.79).

Verified: clean release build, clean DEBUG=1 TEST=1 self-test run (no
asserts), and a fresh head_reference/ checkpoint
(head-reference-20261025 -> 874ef04's regression-model binary) with
sd10/sn5000000 solve counts on the three curated suites essentially
matching the previous table-based checkpoint (70/191 vs 71/191 sd10,
93/191 vs 96/191 sn5M).

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

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