Fix IID's killer-blind gate and its ordering-flag/history contamination

of iValue; harden against a latent ComputeMoveExtension bug.

DO_IID's "is the top move crappy" gate compared raw iValue against
SORT_THESE_FIRST only, missing that ordinary killer moves (FIRST_KILLER
through FOURTH_KILLER) sit below that threshold too -- a killer that
already proved itself elsewhere in the tree was being treated as
"crappy" and triggering an unnecessary shallow rescore. Fixed by also
excluding killer-flagged moves from the gate.

RescoreMovesViaSearch corrupted the winning move's real search score by
OR-ing in SORT_THESE_FIRST to force it to sort first (`mvf[uBest].iValue
|= SORT_THESE_FIRST`) -- unnecessary (SelectBest{With,No}History already
find the true max by plain magnitude comparison, no flag needed) and
actively dangerous: a later ComputeMoveScore() call on that same move,
if it's a capture, would see the corrupted value, mistake it for
generate.c's biased-capture-ordering format, and subtract the wrong
bias entirely. Removed the OR; added an explicit
PLY_INFO.fMovesRescoredByIID flag so ComputeMoveScore and the main
search-loop's move-selection call can both recognize "this ply's
iValue holds a real eval-axis score" without relying on bit-pattern
inference.

Consequently, ComputeMoveScore now trusts an IID-rescored move's score
outright instead of running it through the capture-bias-subtraction or
quiet-move-collapse-to-0 logic (both of which assume generate.c's
ordering encoding, which a rescored ply no longer holds). Separately
hardened it against quiet killer-mate moves, which can reach
SORT_THESE_FIRST via a different, capture-unrelated path and were
incorrectly getting the capture bias subtracted from them; they now
correctly collapse to 0 like other quiet moves.

Two follow-on ideas -- blending history into the real IID score (scaled
or capped) and a exact-tie-only history tiebreak -- were implemented,
measured, and rejected: blending invents a new, leak-prone move-scoring
axis for no measured benefit, and the tiebreak-only compromise still
cost solves relative to just trusting the real score outright. Main
search's move-selection call now branches once per selection (not once
per candidate move) between SelectBestNoHistory (IID-rescored plies)
and SelectBestWithHistory (everyone else), keeping the overwhelmingly
common non-rescored path at zero added cost.

Net measured effect (ecm_ringers.ep_/ecm_confident_quick.ep_/
ecm_hard_quick.ep_, sn=5M): 10/90/9, down from a pre-existing 11/88/10
on ringers and hard specifically -- see lmr_testing/RESULTS.md for the
full sweep of rejected alternatives and why the regression was accepted
as the cost of removing a latent, leak-prone bug class rather than
chasing the exact prior numbers.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01YGSMkwjqiCk4XhbfN7ugD2

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