llama : register the DT3 file type

Adds LLAMA_FTYPE_MOSTLY_DT3 at the end of the ftype enum, the loader
name/guess mappings, the quantization fallbacks (same as the other
ternary types), and the llama-quantize table entry. The table entry
warns that the in-tree quantizer is only the reference one: DT3 models
with the measured quality are produced by the external PTQTP pipeline.

include/llama.h

@@ -156,6 +156,7 @@ extern "C" {
         LLAMA_FTYPE_MOSTLY_NVFP4         = 39, // except 1d tensors
         LLAMA_FTYPE_MOSTLY_Q1_0          = 40, // except 1d tensors
         LLAMA_FTYPE_MOSTLY_Q2_0          = 41, // except 1d tensors
+        LLAMA_FTYPE_MOSTLY_DT3           = 42, // except 1d tensors
 
         LLAMA_FTYPE_GUESSED = 1024, // not specified in the model file
     };