[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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