mtmd: Fix Qwen3-tts-0.6b (#28231)

* mtmd: load the qwen3-tts code predictor proj_in as optional

The talker and the code predictor share the hidden size on the 0.6B
checkpoints, so the reference builds no small_to_mtp_projection and
the conversion emits no tensor for it. The graph already falls back
to identity when the weight is missing, the loader now agrees.

* mtmd: keep the qwen3-tts code predictor ffn_down in F32

The code predictor carries a massive activation: its layer 2 FFN
intermediate peaks around 1.5e5, well past the 65504 ceiling of F16.
mul_mat casts its input to the weight type, so an F16 ffn_down turns
that peak into inf, the residual follows, and the next rms_norm yields
NaN. Reference forward in float32 gives 145109 against 145396 measured
in the graph.

conversion/qwen3tts.py

@@ -276,6 +276,10 @@ class Qwen3TTSSpeakerEncoderModel(MmprojModel):
         # ConvTranspose1d kernels: only F16/F32 are implemented, no BF16
         if new_name.endswith(".conv.weight") and (".up.blk." in new_name or ".dac.blk." in new_name):
             return gguf.GGMLQuantizationType.F32
+        # the code predictor FFN intermediate peaks around 1.5e5, above the F16 range, and mul_mat
+        # casts its input to the weight type
+        if new_name.startswith("a.gen.code.blk.") and new_name.endswith(".ffn_down.weight"):
+            return gguf.GGMLQuantizationType.F32
         return super().tensor_force_quant(name, new_name, bid, n_dims)
 
     @classmethod