mtmd : add MERaLiON-2 multimodal audio support (#21756)

* mtmd : add MERaLiON-2 multimodal audio support

Adds support for A*STAR's MERaLiON-2 audio-language model (3B and 10B)
to the multimodal framework.

Architecture:
- Whisper large-v2 encoder for audio feature extraction
- Gated MLP adaptor: ln_speech -> frame stack (x15) -> Linear+SiLU -> GLU -> out_proj
- Gemma2 3B / 27B decoder

The mmproj GGUF is generated via convert_hf_to_gguf.py --mmproj on the full
MERaLiON-2 model directory (architecture: MERaLiON2ForConditionalGeneration).
The decoder is converted separately as a standard Gemma2 model after stripping
the text_decoder. weight prefix.

New projector type: PROJECTOR_TYPE_MERALION

Supports tasks: speech transcription (EN/ZH/MS/TA), translation, spoken QA.

Model: https://huggingface.co/MERaLiON/MERaLiON-2-3B
       https://huggingface.co/MERaLiON/MERaLiON-2-10B

* simplify comments in meralion adaptor

* meralion: use format_tensor_name, ascii arrows in comments

tools/mtmd/models/whisper-enc.cpp

@@ -95,6 +95,28 @@ ggml_cgraph * clip_graph_whisper_enc::build() {
             FFN_GELU_ERF,
             -1);
 
+    } else if (proj_type == PROJECTOR_TYPE_MERALION) {
+        // stack (above) -> ln -> linear0+silu -> GLU -> out
+        cur = ggml_norm(ctx0, cur, hparams.eps);
+        cur = ggml_mul(ctx0, cur, model.mm_norm_pre_w);
+        cur = ggml_add(ctx0, cur, model.mm_norm_pre_b);
+
+        cur = ggml_mul_mat(ctx0, model.mm_0_w, cur);
+        cur = ggml_add(ctx0, cur, model.mm_0_b);
+        cur = ggml_silu(ctx0, cur);
+
+        ggml_tensor * gate = ggml_mul_mat(ctx0, model.mm_1_w, cur);
+        gate = ggml_add(ctx0, gate, model.mm_1_b);
+        gate = ggml_silu(ctx0, gate);
+
+        ggml_tensor * pool = ggml_mul_mat(ctx0, model.mm_2_w, cur);
+        pool = ggml_add(ctx0, pool, model.mm_2_b);
+
+        cur = ggml_mul(ctx0, gate, pool);
+
+        cur = ggml_mul_mat(ctx0, model.mm_3_w, cur);
+        cur = ggml_add(ctx0, cur, model.mm_3_b);
+
     } else if (proj_type == PROJECTOR_TYPE_GLMA) {
             cur = ggml_norm(ctx0, cur, hparams.eps);
             cur = ggml_mul(ctx0, cur, model.mm_norm_pre_w);