feat(model): secondary unet override slot and ideogram4 native loading

One UNET override cannot serve dual-transformer arches: ideogram4
conditional/unconditional and wan combined-stage experts need separate
files, and previously a single override landed on both experts.

- sd_unet_secondary option with per-slot tracking, consumed-state sync,
  arch-change reset, and incompatible-override fallback
- dropdown renders beside the primary, follows it into quicksettings,
  and is visible only for dual-transformer model types
- ideogram4 native single-file spec with a quant-aware fused-qkv
  converter; such converters run before comfy_quant detection via
  TransformerSpec.converter_handles_quant
- quicksettings render in configured order (sort keyed on the option
  object and always fell back to alphabetical)
- post-load dtype warning skips quantized transformers

modules/ui_definitions.py

@@ -94,6 +94,7 @@ def create_settings(cmd_opts):
         "sd_model_checkpoint": OptionInfo(default_checkpoint, "Base model", DropdownEditable, lambda: {"choices": list_checkpoint_titles()}, refresh=refresh_checkpoints),
         "sd_model_refiner": OptionInfo('None', "Refiner model", gr.Dropdown, lambda: {"choices": ['None'] + list_checkpoint_titles()}, refresh=refresh_checkpoints),
         "sd_unet": OptionInfo("Default", "UNET model", gr.Dropdown, lambda: {"choices": shared_items.sd_unet_items()}, refresh=shared_items.refresh_unet_list),
+        "sd_unet_secondary": OptionInfo("Default", "UNET model secondary", gr.Dropdown, lambda: {"choices": shared_items.sd_unet_items(), "visible": shared_items.sd_unet_secondary_visible()}, refresh=shared_items.refresh_unet_list),
         "latent_history": OptionInfo(20, "Latent history size", gr.Slider, {"minimum": 0, "maximum": 100, "step": 1}),
 
         "advanced_sep": OptionInfo("<h2>Advanced Options</h2>", "", gr.HTML),