refactor(caption): address PR review feedback

Rename WD14 module and settings to WaifuDiffusion:
- Rename wd14.py to waifudiffusion.py
- Rename WD14Tagger class to WaifuDiffusionTagger
- Rename WD14_MODELS constant to WAIFUDIFFUSION_MODELS
- Rename settings: wd14_model -> waifudiffusion_model,
  wd14_character_threshold -> waifudiffusion_character_threshold
- Update all log messages from "WD14" to "WaifuDiffusion"

Code quality improvements:
- Simplify threshold parameter defaulting using `or` operator
- Extract save_output logic into _save_tags_to_file() helper with
  isolated error handling to prevent single file failures from
  impacting entire batch
- Fix timing log format consistency (remove 's' suffix)

modules/interrogate/tagger.py

@@ -1,4 +1,4 @@
-# Unified Tagger Interface - Dispatches to WD14 or DeepBooru based on model selection
+# Unified Tagger Interface - Dispatches to WaifuDiffusion or DeepBooru based on model selection
 # Provides a common interface for the Booru Tags tab
 
 from modules import shared
@@ -7,9 +7,9 @@ DEEPBOORU_MODEL = "DeepBooru"
 
 
 def get_models() -> list:
-    """Return combined list: DeepBooru + WD14 models."""
-    from modules.interrogate import wd14
-    return [DEEPBOORU_MODEL] + wd14.get_models()
+    """Return combined list: DeepBooru + WaifuDiffusion models."""
+    from modules.interrogate import waifudiffusion
+    return [DEEPBOORU_MODEL] + waifudiffusion.get_models()
 
 
 def refresh_models() -> list:
@@ -28,15 +28,15 @@ def load_model(model_name: str) -> bool:
         from modules.interrogate import deepbooru
         return deepbooru.load_model()
     else:
-        from modules.interrogate import wd14
-        return wd14.load_model(model_name)
+        from modules.interrogate import waifudiffusion
+        return waifudiffusion.load_model(model_name)
 
 
 def unload_model():
     """Unload both backends to ensure memory is freed."""
-    from modules.interrogate import deepbooru, wd14
+    from modules.interrogate import deepbooru, waifudiffusion
     deepbooru.unload_model()
-    wd14.unload_model()
+    waifudiffusion.unload_model()
 
 
 def tag(image, model_name: str = None, **kwargs) -> str:
@@ -44,28 +44,28 @@ def tag(image, model_name: str = None, **kwargs) -> str:
 
     Args:
         image: PIL Image to tag
-        model_name: Model to use (DeepBooru or WD14 model name)
+        model_name: Model to use (DeepBooru or WaifuDiffusion model name)
         **kwargs: Additional arguments passed to the backend
 
     Returns:
         Formatted tag string
     """
     if model_name is None:
-        model_name = shared.opts.wd14_model
+        model_name = shared.opts.waifudiffusion_model
 
     if is_deepbooru(model_name):
         from modules.interrogate import deepbooru
         return deepbooru.tag(image, **kwargs)
     else:
-        from modules.interrogate import wd14
-        return wd14.tag(image, model_name=model_name, **kwargs)
+        from modules.interrogate import waifudiffusion
+        return waifudiffusion.tag(image, model_name=model_name, **kwargs)
 
 
 def batch(model_name: str, **kwargs) -> str:
     """Unified batch processing.
 
     Args:
-        model_name: Model to use (DeepBooru or WD14 model name)
+        model_name: Model to use (DeepBooru or WaifuDiffusion model name)
         **kwargs: Additional arguments passed to the backend
 
     Returns:
@@ -75,5 +75,5 @@ def batch(model_name: str, **kwargs) -> str:
         from modules.interrogate import deepbooru
         return deepbooru.batch(model_name=model_name, **kwargs)
     else:
-        from modules.interrogate import wd14
-        return wd14.batch(model_name=model_name, **kwargs)
+        from modules.interrogate import waifudiffusion
+        return waifudiffusion.batch(model_name=model_name, **kwargs)