Tame check-extension compounding; verified neutral in self-play (0.4955)

Fixes a real pathology: checks along a long unbroken forcing line could
extend for free (net zero cost against the qsearch boundary), letting
tree size blow up multiple orders of magnitude on positions like a
near-all-check forced mate (ECM.089: 4.5B nodes / 31min at depth 12
before this change).

- Main-search check extension: gate on SEE soundness (a losing
  sacrifice check gets a small consolation QUARTER_PLY instead of the
  full bonus a sound check gets), and flatten the sound-check bonus to
  a flat THREE_QUARTERS_PLY instead of a near-free ONE_PLY.
- Lower the qsearch entry threshold to match (THREE_QUARTERS_PLY
  instead of ONE_PLY) so a lone check still buys one extra full-width
  ply as before; root.c trims QUARTER_PLY off the per-iteration depth
  budget so this doesn't add a blanket 1/4 ply to every search.
- Qsearch's own check-widening (QSearchFromCheckNoStandPat) now relies
  on fCouldStandPat history plus a g_uIterateDepth/4 ceiling instead of
  an unconditional per-check grant, and QPLIES_OF_NON_CAPTURE_CHECKS
  moved from 1 to 2 to cover both "enter qsearch already in check" and
  "opponent's reply is the first real check" cases with one baseline
  window instead of ad hoc attacker-color tracking.

Net effect on ECM.089 (sn 4M canary): ~7.5x fewer nodes and ~4x less
time at depth 12 versus the original, unbounded behavior. Costs solve
count on the full ECM suite (879 pos, sn 4M): 650 baseline -> 636 here
-- expected and accepted, since ECM is unusually check-extension-heavy
tactics and not representative of real games. Self-play vs baseline
(1000 games, st 1) came back at B_SCORE=0.4955, ELO=-3.1+/-21.5 --
statistically neutral, confirming the fix costs nothing in real play.

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

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