fix(loaders): default control_after_generate to fixed on checkpoint/unet loaders

The previous boolean 'control_after_generate': true defaulted the control
widget to 'randomize', silently changing existing workflows into random
model selection on every queue. A string value sets the default mode, so
'fixed' preserves the prior behavior; users opt into randomization
explicitly.

py/nodes/checkpoint_loader.py

@@ -37,7 +37,7 @@ class CheckpointLoaderLM:
                             "control_after_generate to pick a random model on "
                             "every run."
                         ),
-                        "control_after_generate": True,
+                        "control_after_generate": "fixed",
                     },
                 ),
                 "base_model": (

py/nodes/unet_loader.py

@@ -52,7 +52,7 @@ class UNETLoaderLM:
                             "control_after_generate to pick a random model on "
                             "every run."
                         ),
-                        "control_after_generate": True,
+                        "control_after_generate": "fixed",
                     },
                 ),
                 "weight_dtype": (