metal : add TQ2_0 support (#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

ggml/src/ggml-metal/ggml-metal-impl.h

@@ -87,6 +87,9 @@
 #define N_R0_IQ4_XS 2
 #define N_SG_IQ4_XS 2
 
+#define N_R0_TQ2_0 4
+#define N_SG_TQ2_0 2
+
 // function constants offsets
 #define FC_FLASH_ATTN_EXT_PAD          100
 #define FC_FLASH_ATTN_EXT_BLK          200