Add multilayer neural network

[kevlu8]

Passed STC

Elo   | 8.55 +- 5.00 (95%)
SPRT  | 8.0+0.08s Threads=1 Hash=32MB
LLR   | 3.02 (-2.25, 2.89) [0.00, 5.00]
Games | N: 5652 W: 1488 L: 1349 D: 2815
Penta | [52, 595, 1383, 754, 42]

https://ob.int0x80.ca/test/698/

Passed LTC

Elo   | 16.02 +- 7.06 (95%)
SPRT  | 40.0+0.40s Threads=1 Hash=128MB
LLR   | 2.92 (-2.25, 2.89) [0.00, 5.00]
Games | N: 2540 W: 689 L: 572 D: 1279
Penta | [11, 254, 616, 385, 4]

https://ob.int0x80.ca/test/700/

Bench: 4559824

[Copilot]

Pull request overview

This PR updates the engine’s NNUE evaluation to a multi-layer network with a new SIMD abstraction layer, and adjusts related tuning parameters and accumulator sizing to match the new architecture.

Changes:

• Replace the single hidden-layer NNUE with a 3-layer (L1/L2/L3) network and updated scaling/constants.
• Introduce a SIMD abstraction (simd.hpp + AVX2/AVX512 backends) and rewrite nnue_eval to use it.
• Update accumulator sizing (HL_SIZE → L1_SIZE) and refresh usage in a few call sites; retune various search parameters.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
engine/params.hpp	Updates various tuned search parameters.
engine/nnue/simd.hpp	Declares SIMD abstraction types, constants, and APIs for AVX2/AVX512.
engine/nnue/simd.cpp	Selects the AVX backend implementation to compile.
engine/nnue/avx2.hpp	Implements SIMD abstraction using AVX2 intrinsics.
engine/nnue/avx512.hpp	Implements SIMD abstraction using AVX-512 intrinsics.
engine/nnue/network.hpp	Updates NNUE network layout to multi-layer weights/biases and new sizes.
engine/nnue/network.cpp	Updates network loading and rewrites nnue_eval for multi-layer forward pass.
engine/nnue/accumulator.hpp	Updates accumulator initialization loops to L1_SIZE.
engine/nnue/accumulator.cpp	Updates accumulator update/refresh loops to L1_SIZE.
engine/main.cpp	Updates a refresh call site to use full_refresh(pos, 0).

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.