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/interrogate.py

@@ -21,13 +21,13 @@ def interrogate(image):
         shared.log.debug(f'Interrogate: time={time.time()-t0:.2f} answer="{prompt}"')
         return prompt
     elif shared.opts.interrogate_default_type == 'Tagger':
-        shared.log.info(f'Interrogate: type={shared.opts.interrogate_default_type} model="{shared.opts.wd14_model}"')
+        shared.log.info(f'Interrogate: type={shared.opts.interrogate_default_type} model="{shared.opts.waifudiffusion_model}"')
         from modules.interrogate import tagger
         prompt = tagger.tag(
             image=image,
-            model_name=shared.opts.wd14_model,
+            model_name=shared.opts.waifudiffusion_model,
             general_threshold=shared.opts.tagger_threshold,
-            character_threshold=shared.opts.wd14_character_threshold,
+            character_threshold=shared.opts.waifudiffusion_character_threshold,
             include_rating=shared.opts.tagger_include_rating,
             exclude_tags=shared.opts.tagger_exclude_tags,
             max_tags=shared.opts.tagger_max_tags,