[4/6] Add iterative deepening with aspiration windows and TT move ordering

[luccabb]

Summary

• Implement iterative deepening (search depth 1, 2, 3, ... N)
• Add aspiration windows for faster searches
• Add TT move ordering (best move from previous search tried first)

Details

Iterative Deepening

Instead of searching directly to depth N, we search depth 1, then 2, then 3, etc:

for depth in range(1, target_depth + 1):
    score, move = negamax(board, depth, ...)

Benefits:

• TT entries from depth N-1 improve move ordering at depth N
• Enables future time management (stop when time runs out)
• Killer moves accumulate across iterations

Aspiration Windows

After the first iteration, we use a narrow window around the previous score:

• Initial window: ±50 centipawns
• If search fails high/low, double the window and re-search
• Fall back to full window if window exceeds ±500 cp

This reduces the number of nodes searched when the score is stable.

TT Move Ordering

• Extract best move from TT lookup even if score can't be used
• Put TT move first in the move list before all other moves
• TT move is the best move found at a shallower depth, excellent for ordering

Test plan

• All 64 unit tests pass
• Verified iterative deepening visits all depths

---

🤖 Generated with Claude Code

[github-actions]

🔬 Stockfish Benchmark Results

vs Stockfish Skill Level 3

Metric	Wins	Losses	Draws	Total	Win %
Overall	6	94	0	100	6.0%
As White	3	47	0	50	6.0%
As Black	3	47	0	50	6.0%

vs Stockfish Skill Level 4

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 3-fold repetition: 1

vs Stockfish Skill Level 5

Metric	Wins	Losses	Draws	Total	Win %
Overall	0	98	2	100	0%
As White	0	49	1	50	0%
As Black	0	49	1	50	0%

Non-checkmate endings:

• Draw by 3-fold repetition: 2

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	23349570
Total time	1902.64s
Nodes/second	12272

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=21031      time=1.20s  nps=17502
Position  2/48: nodes=1129860    time=95.64s  nps=11813
Position  3/48: nodes=11903      time=0.65s  nps=18180
Position  4/48: nodes=263861     time=24.22s  nps=10896
Position  5/48: nodes=100749     time=11.54s  nps=8731
Position  6/48: nodes=146608     time=14.18s  nps=10342
Position  7/48: nodes=177580     time=20.56s  nps=8638
Position  8/48: nodes=171084     time=13.55s  nps=12627
Position  9/48: nodes=656782     time=58.41s  nps=11243
Position 10/48: nodes=220167     time=20.03s  nps=10990
Position 11/48: nodes=788160     time=82.85s  nps=9513
Position 12/48: nodes=513731     time=58.67s  nps=8756
Position 13/48: nodes=233763     time=22.63s  nps=10328
Position 14/48: nodes=256929     time=23.64s  nps=10866
Position 15/48: nodes=228144     time=20.25s  nps=11268
Position 16/48: nodes=177053     time=15.38s  nps=11515
Position 17/48: nodes=4593       time=0.28s  nps=16438
Position 18/48: nodes=5341       time=0.26s  nps=20867
Position 19/48: nodes=9642       time=0.63s  nps=15313
Position 20/48: nodes=39555      time=2.43s  nps=16307
Position 21/48: nodes=16248      time=1.09s  nps=14962
Position 22/48: nodes=943        time=0.05s  nps=18459
Position 23/48: nodes=2134       time=0.10s  nps=21611
Position 24/48: nodes=5916       time=0.37s  nps=16132
Position 25/48: nodes=5872       time=0.35s  nps=16559
Position 26/48: nodes=19219      time=1.26s  nps=15282
Position 27/48: nodes=85076      time=6.02s  nps=14128
Position 28/48: nodes=221090     time=18.17s  nps=12170
Position 29/48: nodes=14935      time=1.18s  nps=12672
Position 30/48: nodes=2550       time=0.16s  nps=16115
Position 31/48: nodes=278667     time=23.44s  nps=11889
Position 32/48: nodes=1360147    time=119.25s  nps=11405
Position 33/48: nodes=340668     time=40.60s  nps=8390
Position 34/48: nodes=968806     time=112.84s  nps=8585
Position 35/48: nodes=639806     time=45.48s  nps=14067
Position 36/48: nodes=5977156    time=459.53s  nps=13007
Position 37/48: nodes=8016673    time=568.66s  nps=14097
Position 38/48: nodes=2274       time=0.09s  nps=24671
Position 39/48: nodes=3075       time=0.14s  nps=22137
Position 40/48: nodes=15477      time=0.27s  nps=57476
Position 41/48: nodes=13944      time=0.75s  nps=18542
Position 42/48: nodes=12910      time=0.86s  nps=15041
Position 43/48: nodes=4247       time=0.19s  nps=21932
Position 44/48: nodes=35870      time=2.67s  nps=13453
Position 45/48: nodes=46602      time=3.61s  nps=12894
Position 46/48: nodes=102729     time=8.54s  nps=12031
Position 47/48: nodes=0          time=0.00s  nps=0  (terminal)
Position 48/48: nodes=0          time=0.00s  nps=0  (terminal)