metal : add TQ2_0 support (llama/26980)

metal : add TQ2_0 support (llama/26980)

* metal: add TQ2_0 support

Add support for the GGML_TYPE_TQ2_0 (ternary, 2 bits per element) type in
the Metal backend.

Assisted-by: llama.cpp:DeepSeek-v4-Flash-0731
* cont : optimize mul_mv kernel

- float ops over integer ops
- precalculate sums
- hoist coef out of the inner loop
- contiguous y loads

llama.cpp:DeepSeek-v4-Flash-0731