[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

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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	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