mtmd: add load progress callback

[ngxson]

Overview

cont #24828

example output:

data: {"model":"unsloth/Qwen3.6-35B-A3B-MTP-GGUF:Q4_K_M","event":"model_status","data":{"status":"loading"}}

data: {"model":"unsloth/Qwen3.6-35B-A3B-MTP-GGUF:Q4_K_M","event":"status_change","data":{"status":"loading","progress":{"stage":"fit_params"}}}

data: {"model":"unsloth/Qwen3.6-35B-A3B-MTP-GGUF:Q4_K_M","event":"status_change","data":{"status":"loading","progress":{"stage":"text_model","value":0.0}}}

data: {"model":"unsloth/Qwen3.6-35B-A3B-MTP-GGUF:Q4_K_M","event":"status_change","data":{"status":"loading","progress":{"stage":"text_model","value":1.0}}}

data: {"model":"unsloth/Qwen3.6-35B-A3B-MTP-GGUF:Q4_K_M","event":"status_change","data":{"status":"loading","progress":{"stage":"mmproj_model","value":0.0}}}

data: {"model":"unsloth/Qwen3.6-35B-A3B-MTP-GGUF:Q4_K_M","event":"status_change","data":{"status":"loading","progress":{"stage":"mmproj_model","value":0.8644336462020874}}}

data: {"model":"unsloth/Qwen3.6-35B-A3B-MTP-GGUF:Q4_K_M","event":"status_change","data":{"status":"loading","progress":{"stage":"mmproj_model","value":1.0}}}

data: {"model":"unsloth/Qwen3.6-35B-A3B-MTP-GGUF:Q4_K_M","event":"status_change","data":{"status":"loaded","info":{"id":"unsloth/Qwen3.6-35B-A3B-MTP-GGUF:Q4_K_M","aliases":["unsloth/Qwen3.6-35B-A3B-MTP-GGUF:Q4_K_M"],"tags":[],"object":"model","created":1782037899,"owned_by":"llamacpp","meta":{"vocab_type":2,"n_vocab":248320,"n_ctx":262144,"n_ctx_train":262144,"n_embd":2048,"n_params":35505251456,"size":22652396032}}}}

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure: mostly for auto-completion