feat(grading): record applied params in postprocess metadata

Color grading mutated pp.image but never wrote to pp.info, so the
chosen brightness/contrast/gamma/LUT/etc. were invisible in the
saved file's postprocessing/extras chunks and in the live Output
panel — unlike every other postprocessor (Upscale, Rembg, NudeNet,
PixelArt) which all stamp their settings.

Stamp every non-default GradingParams field as 'Grading <name>' so
the trail is consistent with the rest of the Process tab. LUT file
is recorded by basename rather than full path to keep the metadata
string compact.

scripts/color_grading.py

@@ -1,3 +1,5 @@
+import os
+from dataclasses import fields
 from modules import scripts_postprocessing, ui_sections, processing_grading
 
 
@@ -12,5 +14,14 @@ class ScriptPostprocessingColorGrading(scripts_postprocessing.ScriptPostprocessi
     def process(self, pp: scripts_postprocessing.PostprocessedImage, *args, **kwargs): # pylint: disable=arguments-differ
         kwargs = {k: v for k, v in kwargs.items() if v is not None}
         grading_params = processing_grading.GradingParams(*args, **kwargs)
-        if processing_grading.is_active(grading_params):
-            pp.image = processing_grading.grade_image(pp.image, grading_params)
+        if not processing_grading.is_active(grading_params):
+            return
+        pp.image = processing_grading.grade_image(pp.image, grading_params)
+        defaults = processing_grading.GradingParams()
+        for f in fields(grading_params):
+            val = getattr(grading_params, f.name)
+            if val == getattr(defaults, f.name):
+                continue
+            if f.name == 'lut_cube_file' and val:
+                val = os.path.basename(str(val))
+            pp.info[f"Grading {f.name.replace('_', ' ')}"] = val