Fix: ensure bitnet-lut-kernels.h prepared before llama.cpp build

[shivansh31414]

Title: Fix CMake build: ensure bitnet-lut-kernels.h prepared before llama.cpp

Summary
The CMake build currently fails because include/bitnet-lut-kernels.h is required at configure time but is not tracked in the source tree. This header is produced by setup scripts (setup_env.py, codegen utilities) and not by CMake itself, which breaks a clean cmake .. && make workflow.

Problem

• bitnet-lut-kernels.h is missing during CMake configure.
• ggml’s CMakeLists expects the header before add_subdirectory(3rdparty/llama.cpp).
• Contributors running plain CMake builds hit a fatal error without running setup scripts first.

Fix Implemented
fixes: #378

• Updated top‑level CMakeLists.txt to ensure include/bitnet-lut-kernels.h exists before llama.cpp is added.
• Added two cache variables:
  • BITNET_PRESET_KERNEL_MODEL (default: bitnet_b1_58-3B)
  • BITNET_LUT_KERNELS_FILE (optional explicit header path)
• Behavior:
  • If header already exists → leave untouched.
  • If BITNET_LUT_KERNELS_FILE is set → copy that file.
  • Else → auto‑copy from preset_kernels/<model>/bitnet-lut-kernels-{tl1|tl2}.h based on architecture.
  • If nothing valid is found → clear fatal error with guidance.

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[Copilot]

Pull request overview

This PR fixes clean CMake configure/build failures by ensuring include/bitnet-lut-kernels.h is present before 3rdparty/llama.cpp / ggml is configured, using either a user-provided header path or a preset-kernel header selected by model + architecture.

Changes:

• Adds CMake cache variables BITNET_PRESET_KERNEL_MODEL and BITNET_LUT_KERNELS_FILE to control how bitnet-lut-kernels.h is prepared.
• Implements configure-time logic to copy a provided header or a preset header into include/bitnet-lut-kernels.h before adding llama.cpp.
• Adds a clearer fatal error when no suitable preset header can be found.

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