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/shared_items.py

@@ -116,6 +116,12 @@ def refresh_unet_list():
     modules.sd_unet.refresh_unet_list()
 
 
+def sd_unet_secondary_visible():
+    import modules.sd_unet
+    from modules import shared
+    return shared.sd_model_type in modules.sd_unet.DUAL_TRANSFORMER_TYPES
+
+
 def sd_te_items():
     import modules.model_te
     predefined = ['Default']