Search: add capture history without changing move stages

[sebhofmann]

Closes #9

Summary

• add capture history indexed by moving piece, destination, and captured piece type
• reward the cutoff capture and apply gravity maluses to previously searched captures
• use a bounded capture-history term only within the existing good/bad capture ordering
• report capture-history range, population, and saturation under DEBUG_HISTORY_RANGE

The stage order remains TT -> good captures -> quiets -> bad captures. Evasions and pruning decisions do not consume capture history.

Validation

• make clean && make both
• ./perft_test.sh: 29 passed, 0 failed (4 slow cases skipped)
• ./see_test.sh: 92 passed, 0 failed
• make test-nnue: passed
• capture-history diagnostics: [-565, 24435], 100 nonzero, 0 saturated; then [-5032, 22887], 171 nonzero, 0 saturated
• ./symmetry_test.sh: evaluation checks pass; the pre-existing search-symmetry suite still reports its known search-bias failures

LTC SPRT (paused; inconclusive)

Per request, the run was stopped and will be continued tomorrow. This is not an accepted Elo gain.

• Command: TC=60+0.6 ./tournament.sh sprt 9_new 9_base
• Openings: /home/paschty/Downloads/2moves_v2.pgn
• Paired colors: -games 2 -repeat
• H0/H1: 0 / +5 Elo; alpha=beta=0.05
• Current W/L/D: 40 / 40 / 411 (491 games)
• Last complete rating report (at 490 games): +0.7 +/- 12.3 Elo
• LLR: -0.224, bounds [-2.94, +2.94] — inconclusive
• Base commit: 056488eef9b51b70cb4b826ed028b280afd870fc
• Test commit: 05bd33ff08f32456b860063f9de5e69c98da3207

The local snapshots 9_base and 9_new, PGN, and 9_sprt.log are preserved for continuation.

[sebhofmann]

LTC SPRT run 2 — manually stopped, not accepted

The restarted LTC run was manually stopped as requested. It did not reach either SPRT boundary and is therefore inconclusive. The negative point estimate and LLR must not be presented as an accepted Elo gain. The PR remains open.

• command: TC=60+0.6 ./tournament.sh sprt 9_new 9_base
• openings: /home/paschty/Downloads/2moves_v2.pgn
• paired colors: yes (-games 2 -repeat)
• H0/H1: 0 / +5 Elo
• alpha/beta: 0.05 / 0.05
• base commit: 056488eef9b51b70cb4b826ed028b280afd870fc
• test commit: 05bd33ff08f32456b860063f9de5e69c98da3207
• final W/L/D: 108 / 124 / 949 (1,181 completed games)
• Elo estimate: -4.7 +/- 8.8 Elo
• LOS: 14.7%
• draw ratio: 80.4%
• LLR: -1.79, bounds [-2.94, +2.94]
• decision: inconclusive / manually stopped; not accepted

Artifacts are preserved locally as:

• sprt_archive/issue_9/9_sprt_run2_1181games.log
• sprt_archive/issue_9/9_sprt_run2_1181games.pgn

Run 1 remains archived separately at 491 games.