Replace SEARCH_SORT_LIMIT's pure move-count cutoff with a quality-based

one: always fully select "high performer" moves regardless of count,
only apply the per-ply budget to leftover ordinary moves.

The old gate (uLegalMoves < SEARCH_SORT_LIMIT(ply)) stopped selecting
carefully after a fixed count, counting the hash move too -- so at
ply 6+ (limit 5), a position with a hash move already used one of only
5 total slots before the cutoff hit. It had no way to tell "a handful
of mediocre quiet moves" from "a hash move plus three winning captures
and two killers" -- in the latter case, a real high-performer beyond
the 4th/5th slot would get treated identically to a random leftover
quiet move, even though generate.c had already tagged it as excellent.

Checked what three real engines do here: Crafty always fully sorts the
hash move, then MVV/LVA-ordered captures, then up to 4 killers -- its
own cheap fallback (a move-count cutoff, gated by remaining depth) only
ever applies to what's left after all of that, i.e. plain untested
quiet moves. Stockfish uses a value threshold, not a position/count
threshold, so a good move is never orphaned by where it happens to sit
in the list, only by its own assessed quality. Berserk never gates at
all -- full selection sort unconditionally, every node.

New design: keep fully selecting for as long as every move found so
far is >= GOOD_MOVE (a generate.c ordering-encoding constant that
already sits, by construction, below every killer tier and
SORT_THESE_FIRST's winning/even-capture range, and above ordinary
quiet moves and losing captures -- a real quality floor already baked
into the existing encoding, not a new one). The first selection that
reveals a move below that floor marks the transition to "the rest of
the team"; from there, SEARCH_SORT_LIMIT's existing table is reused
(as an explicitly untuned starting point -- its old numbers were
calibrated, if at all, against a different question: total selection
budget from move 1, not a leftover-only budget) to decide how many
more full selections are worth the cost before taking the remainder in
place. On an IID-rescored ply, GOOD_MOVE is meaningless (iValue is a
real eval-axis score there, not generate.c's encoding), so that ply
type keeps its existing unconditional full-select behavior unchanged.

Measured (ecm_ringers.ep_/ecm_confident_quick.ep_/ecm_hard_quick.ep_,
sn=5M) against the prior baseline (10/88/9): 11/87/12, net +3 solves.
EBF: unchanged on ringers, worse on confident_quick (the one suite that
also lost a solve -- consistent single-suite regression, not a
systemic pattern), better on hard_quick (paired with its solve gain).
Not yet a fully validated result -- SEARCH_SORT_LIMIT's numbers
(17/12/9/7/6/5) now need their own recalibration pass under this new
"leftover budget" meaning, since whatever they were tuned against
before doesn't apply to this role.

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

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