[3/6] Add MVV-LVA capture ordering and killer moves heuristic

[luccabb]

Summary

• Sort captures by MVV-LVA (Most Valuable Victim - Least Valuable Attacker)
• Add killer moves heuristic for quiet move ordering
• Track quiet moves that cause beta cutoffs (2 per ply)

Details

MVV-LVA Capture Ordering

Captures are now sorted by MVV-LVA score, putting the best captures first:

• Taking a queen with a pawn is searched before taking a pawn with a queen
• Increases the rate of beta cutoffs, reducing nodes searched

Killer Moves Heuristic

Quiet moves that caused beta cutoffs are remembered:

• Each ply depth keeps track of 2 killer moves
• Killers are searched immediately after captures
• Helps find refutation moves faster in similar positions

Move Ordering Priority

1. Captures - sorted by MVV-LVA
2. Killer moves - quiet moves that caused cutoffs at this ply
3. Other quiet moves - shuffled for variety

Test plan

• All 64 unit tests pass
• Verified killer moves are updated on beta cutoffs

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🤖 Generated with Claude Code

[github-actions]

🔬 Stockfish Benchmark Results

vs Stockfish Skill Level 3

Metric	Wins	Losses	Draws	Total	Win %
Overall	14	85	1	100	14.0%
As White	7	42	1	50	14.0%
As Black	7	43	0	50	14.0%

Non-checkmate endings:

• Draw by 3-fold repetition: 1

vs Stockfish Skill Level 4

Metric	Wins	Losses	Draws	Total	Win %
Overall	12	85	3	100	12.0%
As White	7	42	1	50	14.0%
As Black	5	43	2	50	10.0%

Non-checkmate endings:

• Draw by insufficient mating material: 1
• Draw by 3-fold repetition: 1

vs Stockfish Skill Level 5

Metric	Wins	Losses	Draws	Total	Win %
Overall	2	97	1	100	2.0%
As White	2	47	1	50	4.0%
As Black	0	50	0	50	0%

Non-checkmate endings:

• Draw by insufficient mating material: 1

Configuration

• 5 chunks × 20 rounds × 3 skill levels = 300 total games
• Each opening played with colors reversed (-repeat) for fairness
• Moonfish: 60s per move
• Stockfish: 60+5 time control

[github-actions]

🔬 Stockfish Benchmark Results

vs Stockfish Skill Level 3

Metric	Wins	Losses	Draws	Total	Win %
Overall	7	91	2	100	7.0%
As White	6	42	2	50	12.0%
As Black	1	49	0	50	2.0%

Non-checkmate endings:

• Draw by fifty moves rule: 1
• Draw by 3-fold repetition: 1

vs Stockfish Skill Level 4

Metric	Wins	Losses	Draws	Total	Win %
Overall	11	86	3	100	11.0%
As White	7	41	2	50	14.0%
As Black	4	45	1	50	8.0%

Non-checkmate endings:

• Draw by 3-fold repetition: 2

vs Stockfish Skill Level 5

Metric	Wins	Losses	Draws	Total	Win %
Overall	2	96	2	100	2.0%
As White	1	48	1	50	2.0%
As Black	1	48	1	50	2.0%

Non-checkmate endings:

• Draw by fifty moves rule: 1
• Draw by 3-fold repetition: 1

Configuration

• 5 chunks × 20 rounds × 3 skill levels = 300 total games
• Each opening played with colors reversed (-repeat) for fairness
• Moonfish: 60s per move
• Stockfish: 60+5 time control

[luccabb]

/run-nps-benchmark

[github-actions]

⚡ NPS Benchmark Results

Metric	Value
Depth	5
Positions	48
Total nodes	31699656
Total time	2965.32s
Nodes/second	10690

Node count is the primary signal — it's deterministic and catches search behavior changes. If the node count changes, the PR changed search behavior. NPS is informational only (CI runner performance varies).

Per-position breakdown

Position  1/48: nodes=50546      time=3.69s  nps=13681
Position  2/48: nodes=3479582    time=359.44s  nps=9680
Position  3/48: nodes=6513       time=0.39s  nps=16632
Position  4/48: nodes=1514766    time=152.66s  nps=9922
Position  5/48: nodes=166003     time=18.26s  nps=9090
Position  6/48: nodes=502188     time=50.61s  nps=9923
Position  7/48: nodes=308775     time=28.84s  nps=10706
Position  8/48: nodes=286464     time=22.65s  nps=12649
Position  9/48: nodes=988200     time=78.11s  nps=12651
Position 10/48: nodes=846822     time=76.68s  nps=11043
Position 11/48: nodes=863866     time=92.90s  nps=9299
Position 12/48: nodes=648389     time=59.86s  nps=10831
Position 13/48: nodes=349739     time=30.74s  nps=11375
Position 14/48: nodes=377751     time=34.25s  nps=11028
Position 15/48: nodes=375960     time=28.70s  nps=13100
Position 16/48: nodes=833014     time=71.93s  nps=11581
Position 17/48: nodes=14449      time=0.99s  nps=14592
Position 18/48: nodes=4261       time=0.22s  nps=19804
Position 19/48: nodes=14203      time=1.09s  nps=12987
Position 20/48: nodes=39479      time=2.29s  nps=17214
Position 21/48: nodes=14797      time=0.85s  nps=17380
Position 22/48: nodes=556        time=0.03s  nps=20239
Position 23/48: nodes=4169       time=0.24s  nps=17591
Position 24/48: nodes=9828       time=0.57s  nps=17273
Position 25/48: nodes=10090      time=0.56s  nps=18095
Position 26/48: nodes=52432      time=3.79s  nps=13821
Position 27/48: nodes=128871     time=8.60s  nps=14978
Position 28/48: nodes=635469     time=52.57s  nps=12087
Position 29/48: nodes=81118      time=6.80s  nps=11924
Position 30/48: nodes=1117       time=0.07s  nps=16533
Position 31/48: nodes=943797     time=82.67s  nps=11416
Position 32/48: nodes=1307463    time=115.21s  nps=11348
Position 33/48: nodes=940285     time=115.94s  nps=8110
Position 34/48: nodes=1877969    time=203.71s  nps=9218
Position 35/48: nodes=1037438    time=83.52s  nps=12421
Position 36/48: nodes=7001400    time=677.40s  nps=10335
Position 37/48: nodes=5069942    time=430.05s  nps=11789
Position 38/48: nodes=2396       time=0.11s  nps=21984
Position 39/48: nodes=11198      time=0.40s  nps=27659
Position 40/48: nodes=12236      time=0.22s  nps=55875
Position 41/48: nodes=32999      time=2.14s  nps=15415
Position 42/48: nodes=35709      time=2.24s  nps=15928
Position 43/48: nodes=3977       time=0.20s  nps=19922
Position 44/48: nodes=58484      time=4.36s  nps=13417
Position 45/48: nodes=43661      time=3.30s  nps=13232
Position 46/48: nodes=711285     time=55.46s  nps=12824
Position 47/48: nodes=0          time=0.00s  nps=0  (terminal)
Position 48/48: nodes=0          time=0.00s  nps=0  (terminal)