[2/6] Fix transposition table with proper bounds

[luccabb]

Summary

• Add Bound enum (EXACT, LOWER_BOUND, UPPER_BOUND) for correct TT behavior
• Use Zobrist hash as cache key instead of FEN string (faster)
• Store bound type and depth with each cache entry
• Fix null move pruning condition (was missing null_move parameter check)
• Update parallel engines to use new cache format

Details

The transposition table now correctly stores and uses bound information:

• EXACT: Score is within the alpha-beta window, can be used directly
• LOWER_BOUND: Node failed high (score >= beta), true score is at least this value
• UPPER_BOUND: Node failed low (score <= alpha), true score is at most this value

This prevents incorrect cutoffs and score usage that can cause search instability.

Parallel Engine Updates

All parallel engines (lazy_smp, l1p, l2p) updated to:

• Use Zobrist hash for cache key lookup
• Use context managers for proper Pool/Manager cleanup
• Fix score negation in l1p (opponent perspective -> our perspective)

Test plan

• All 64 unit tests pass
• Verified correct bound handling in search

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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	10	88	2	100	10.0%
As White	7	42	1	50	14.0%
As Black	3	46	1	50	6.0%

Non-checkmate endings:

• Draw by 3-fold repetition: 2

vs Stockfish Skill Level 4

Metric	Wins	Losses	Draws	Total	Win %
Overall	7	91	2	100	7.0%
As White	3	46	1	50	6.0%
As Black	4	45	1	50	8.0%

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 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	25043687
Total time	2651.28s
Nodes/second	9445

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=208940     time=17.67s  nps=11826
Position  2/48: nodes=1105550    time=108.17s  nps=10220
Position  3/48: nodes=19218      time=1.46s  nps=13201
Position  4/48: nodes=1317286    time=151.81s  nps=8676
Position  5/48: nodes=159788     time=19.69s  nps=8114
Position  6/48: nodes=463056     time=49.91s  nps=9278
Position  7/48: nodes=226632     time=27.77s  nps=8161
Position  8/48: nodes=671582     time=59.87s  nps=11217
Position  9/48: nodes=1911870    time=178.56s  nps=10707
Position 10/48: nodes=499511     time=54.74s  nps=9124
Position 11/48: nodes=644015     time=77.83s  nps=8274
Position 12/48: nodes=1140029    time=135.25s  nps=8428
Position 13/48: nodes=1175548    time=126.79s  nps=9271
Position 14/48: nodes=815258     time=86.78s  nps=9394
Position 15/48: nodes=974275     time=98.08s  nps=9933
Position 16/48: nodes=465441     time=41.33s  nps=11261
Position 17/48: nodes=13922      time=1.01s  nps=13777
Position 18/48: nodes=8801       time=0.45s  nps=19349
Position 19/48: nodes=37410      time=2.92s  nps=12816
Position 20/48: nodes=75661      time=4.89s  nps=15472
Position 21/48: nodes=54866      time=4.51s  nps=12152
Position 22/48: nodes=572        time=0.03s  nps=20067
Position 23/48: nodes=6634       time=0.42s  nps=15741
Position 24/48: nodes=50071      time=3.87s  nps=12941
Position 25/48: nodes=9154       time=0.57s  nps=16071
Position 26/48: nodes=75947      time=6.33s  nps=12003
Position 27/48: nodes=150123     time=13.07s  nps=11484
Position 28/48: nodes=552794     time=51.52s  nps=10728
Position 29/48: nodes=244226     time=26.51s  nps=9212
Position 30/48: nodes=2752       time=0.22s  nps=12696
Position 31/48: nodes=1655450    time=163.17s  nps=10145
Position 32/48: nodes=1276259    time=121.18s  nps=10532
Position 33/48: nodes=1901722    time=303.30s  nps=6270
Position 34/48: nodes=1357225    time=186.35s  nps=7283
Position 35/48: nodes=765752     time=70.38s  nps=10879
Position 36/48: nodes=2475889    time=232.70s  nps=10640
Position 37/48: nodes=1433122    time=123.89s  nps=11568
Position 38/48: nodes=10881      time=0.52s  nps=21021
Position 39/48: nodes=8814       time=0.32s  nps=27770
Position 40/48: nodes=22399      time=0.44s  nps=51445
Position 41/48: nodes=29056      time=2.09s  nps=13899
Position 42/48: nodes=78302      time=5.63s  nps=13903
Position 43/48: nodes=10814      time=0.63s  nps=17272
Position 44/48: nodes=97438      time=8.46s  nps=11523
Position 45/48: nodes=82875      time=7.17s  nps=11556
Position 46/48: nodes=756757     time=73.03s  nps=10361
Position 47/48: nodes=0          time=0.00s  nps=0  (terminal)
Position 48/48: nodes=0          time=0.00s  nps=0  (terminal)