mtmd: add Gemma 4 audio conformer encoder support (#21421)

* mtmd: add Gemma 4 audio conformer encoder support

Add audio processing for Gemma 4 E2B/E4B via a USM-style Conformer.

Architecture:
- 12-layer Conformer: FFN → Self-Attention → Causal Conv1D → FFN → Norm
- Subsampling Conv Projection: 2x Conv2D(stride=2) with LayerNorm
- Full self-attention with sinusoidal RPE and sliding window mask (24)
- Logit softcapping at 50.0, ClippableLinear clamping
- Output: 1024 → 1536 → RMSNorm → multimodal embedder

Mel preprocessing (dedicated mtmd_audio_preprocessor_gemma4a):
- HTK mel scale, 128 bins, magnitude STFT, mel_floor=1e-3
- Standard periodic Hann window (320 samples), zero-padded to FFT size
- Semicausal left-padding (frame_length/2 samples)
- Frame count matched to PyTorch (unfold formula)
- No pre-emphasis, no Whisper-style normalization
- Mel cosine similarity vs PyTorch: 0.9998

Key fixes:
- Tensor loading dedup: prevent get_tensor() from creating duplicate
  entries in ctx_data. Fixed with std::set guard.
- ClippableLinear clamp_info loading moved after per-layer tensors.
- Sliding window mask (24 positions) matching PyTorch context_size.
- Skip Whisper normalization for Gemma4 mel output.

Tested on E2B and E4B with CPU and Vulkan backends.
Transcribes: "Glad to see things are going well and business is starting
to pick up" (matching ground truth).

Ref: #21325

tools/mtmd/clip-model.h

@@ -217,6 +217,13 @@ struct clip_layer {
     ggml_tensor * conv_pw2_w    = nullptr;
     ggml_tensor * conv_pw2_b    = nullptr;
 
+    // gemma4 audio conformer per-layer
+    ggml_tensor * attn_pre_norm_w   = nullptr;
+    ggml_tensor * attn_k_rel_w      = nullptr;
+    ggml_tensor * per_dim_scale_w   = nullptr;
+    ggml_tensor * per_dim_k_scale_w = nullptr;
+    ggml_tensor * ff_post_norm_1_w  = nullptr;
+
     bool has_deepstack() const {
         return deepstack_fc1_w != nullptr;
     }
@@ -459,6 +466,15 @@ struct clip_model {
     };
     std::map<std::string, clamp_info> clamp_info_map;
 
+    // gemma4 audio conformer
+    std::array<ggml_tensor *, 2> sscp_conv_w = {nullptr};
+    std::array<ggml_tensor *, 2> sscp_conv_b = {nullptr};
+    std::array<ggml_tensor *, 2> sscp_norm_w = {nullptr};
+    ggml_tensor * sscp_inp_proj_w = nullptr;
+    ggml_tensor * sscp_inp_proj_b = nullptr;
+    ggml_tensor * audio_out_proj_w = nullptr;
+    ggml_tensor * audio_out_proj_b = nullptr;
+
     bool audio_has_avgpool() const {
         return proj_type == PROJECTOR_TYPE_QWEN2A
             || proj_type == PROJECTOR_TYPE_VOXTRAL