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/mtmd.cpp

@@ -476,6 +476,7 @@ struct mtmd_context {
                 } break;
             case PROJECTOR_TYPE_ULTRAVOX:
             case PROJECTOR_TYPE_GLMA:
+            case PROJECTOR_TYPE_MERALION:
                 {
                     audio_preproc = std::make_unique<mtmd_audio_preprocessor_whisper>(ctx_a);
                 } break;