[WIP] Cleanup and reorganize MetalFish codebase

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This section details on the original issue you should resolve

<issue_title>[Chore] Codebase cleanup, reorganization, and dead code removal</issue_title>
<issue_description>## Summary

Comprehensive cleanup and reorganization of the MetalFish codebase to improve maintainability, reduce redundancy, and establish a cleaner architecture. This involves analyzing every file, removing unused code, merging overlapping functionality, restructuring directories, and removing all references to Stockfish and Lc0.

Current State Analysis

Directory Structure (Current)

src/
├── core/          # 7,554 lines - Chess fundamentals
├── eval/          # 5,470 lines - NNUE evaluation
├── gpu/           # 7,878 lines - GPU backends (Metal, CUDA, CPU)
├── mcts/          # 9,160 lines - MCTS implementations (MULTIPLE!)
├── search/        # 5,195 lines - Alpha-beta search
├── syzygy/        # 1,780 lines - Tablebase probing
├── uci/           # 3,591 lines - UCI protocol
├── main.cpp
├── benchmark_gpu.cpp
└── paper_benchmark.cpp

tests/
├── 19 test files with overlapping coverage
└── ~40,000 lines total

Key Issues Identified

1. Multiple MCTS Implementations (HIGH PRIORITY)

The src/mcts/ directory has overlapping implementations:

File	Lines	Purpose	Status
thread_safe_mcts.cpp/h	2,359	Primary MCTS	✅ Keep
apple_silicon_mcts.cpp/h	1,323	Apple-specific MCTS	⚠️ Merge into thread_safe
parallel_hybrid_search.cpp/h	1,907	Hybrid AB+MCTS	⚠️ Review for merge
ab_integration.cpp/h	1,462	AB integration	⚠️ Merge with parallel_hybrid
lc0_mcts_core.h	1,077	Lc0 types (header-only)	⚠️ Review necessity
stockfish_adapter.cpp/h	693	SF compatibility	⚠️ Rename/rebrand
position_classifier.cpp/h	871	Position analysis	✅ Keep

Problem: AppleSiliconMCTSEvaluator, ThreadSafeMCTS, and ParallelHybridSearch all implement similar MCTS logic with different optimizations. This should be ONE configurable class.

2. Redundant Test Files

Multiple test files cover the same functionality:

Redundant Pair	Action
test_core.cpp + test_bitboard.cpp + test_position.cpp + test_movegen.cpp	Merge into test_core.cpp
test_mcts.cpp + test_mcts_module.cpp + test_mcts_comprehensive.cpp	Merge into test_mcts.cpp
test_search.cpp + test_search_module.cpp + test_search_comprehensive.cpp	Merge into test_search.cpp
test_gpu_nnue.cpp + test_gpu_module.cpp + test_gpu_comprehensive.cpp	Merge into test_gpu.cpp
test_hybrid.cpp + test_hybrid_comprehensive.cpp	Merge into test_hybrid.cpp

3. Large Files Needing Split

File	Lines	Recommendation
gpu_nnue_integration.cpp	2,313	Split: weights, inference, batching
search.cpp	2,191	Split: search, aspiration, pruning
uci.cpp	1,974	Split: parsing, commands, options
thread_safe_mcts.cpp	1,634	Split: tree, search, evaluation

4. Platform-Specific Code Not Properly Isolated

Files with mixed platform code that should be separated:

• src/core/memory.cpp - Has #ifdef __APPLE__ mixed in
• src/core/numa.h - 1,437 lines with platform-specific sections
• src/search/apple_silicon_search.h - Should be in a platform/ directory
• src/mcts/apple_silicon_mcts.cpp - Should be in platform/

5. Unused/Dead Code Patterns

• src/core/shm.h and src/core/shm_linux.h - Shared memory (likely unused)
• src/core/perft.h - Perft testing (move to tests/)
• src/search/tune.cpp/h - Tuning infrastructure (review if used)
• src/gpu/cpu_backend.cpp - CPU fallback (verify it's actually used)

6. Stockfish/Lc0 References That Must Be Removed (HIGH PRIORITY)

MetalFish is its own engine. All references to Stockfish and Lc0 must be removed or rebranded.

Files with Stockfish references (18 files):

• src/gpu/gpu_accumulator_cache.h
• src/gpu/gpu_nnue_integration.cpp
• src/gpu/gpu_mcts_backend.h
• src/gpu/gpu_mcts_backend.cpp
• src/gpu/gpu_nnue_integration.h
• src/core/numa.h
• src/uci/engine.h
• src/uci/benchmark.cpp
• src/uci/uci.cpp
• src/eval/nnue/network.cpp
• src/mcts/thread_safe_mcts.cpp
• src/mcts/parallel_hybrid_search.h
• src/mcts/parallel_hybrid_search.cpp
• src/mcts/stockfish_adapter.h → Rename to metalfish_adapter.h
• src/mcts/ab_integration.h
• src/mcts/ab_integration.cpp
• src/mcts/stockfish_adapter.cpp → Rename to metalfish_adapter.cpp
• tests/test_hybrid.cpp

Files with Lc0/Leela references (8 files):

• src/mcts/apple_silicon_mcts.h
• src/mcts/thread_safe_mcts.h
• src/mcts/apple_silicon_mcts.cpp
• src/mcts/thread_safe_mcts.cpp
• src/mcts/lc0_mcts_core.h → Rename to mcts_core.h
• src/mcts/parallel_hybrid_search.cpp
• src/mcts/stockfish_adapter.h
• `src/mcts/stockfish_adapter...

• Fixes [Chore] Codebase cleanup, reorganization, and dead code removal #18

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