Add Q4_K/Q5_K/Q6_K GPU support via Q8_0 dequantization

[AdamBien]

• Add GPU support for K-quant models (Q4_K_M, Q5_K_M, Q6_K) via load-time dequantization to Q8_0
• New FloatTensor implementations: Q4_KFloatTensor, Q5_KFloatTensor, Q6_KFloatTensor
• Dequantization correctly handles TornadoVM's 16-byte ARRAY_HEADER memory layout
• Centralize weight loading log message in AbstractModelLoader (shows actual model quantization, e.g. "Q4_K_M -> Q8_0")

Tested with:
./llamaTornado --gpu --verbose-init --metal --model /Users/abien/work/workspaces/llms/Devstral-Small-2-24B-Instruct-2512-Q4_K_M.gguf --prompt "who are you?" --gpu-memory 30GB

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