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/clip-impl.h

@@ -259,6 +259,7 @@ enum projector_type {
     PROJECTOR_TYPE_GLMA,
     PROJECTOR_TYPE_QWEN25O, // will be replaced by QWEN2A or QWEN25VL depending on clip_ctx
     PROJECTOR_TYPE_VOXTRAL,
+    PROJECTOR_TYPE_MERALION,
     PROJECTOR_TYPE_MUSIC_FLAMINGO,
     PROJECTOR_TYPE_LFM2,
     PROJECTOR_TYPE_KIMIVL,
@@ -302,6 +303,7 @@ static std::map<projector_type, std::string> PROJECTOR_TYPE_NAMES = {
     { PROJECTOR_TYPE_GLMA,      "glma"},
     { PROJECTOR_TYPE_QWEN25O,   "qwen2.5o"},
     { PROJECTOR_TYPE_VOXTRAL,   "voxtral"},
+    { PROJECTOR_TYPE_MERALION,  "meralion"},
     { PROJECTOR_TYPE_MUSIC_FLAMINGO, "musicflamingo"},
     { PROJECTOR_TYPE_LFM2,      "lfm2"},
     { PROJECTOR_TYPE_KIMIVL,    "kimivl"},