test: gitignore FP8 quantization sample PNGs written to the repo root

tests/diffusion/quantization/test_quantization_fp8.py saves test_*_fp8.png
into the working directory for manual inspection; they are byproducts that
kept showing up as untracked files. Ignore them so they are never committed.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitignore

@@ -275,3 +275,8 @@ tests/.ci_generated/
 
 # Local Lance precision-matching dump scripts (not part of the repo)
 lance_compare/
+
+# FP8 quantization test sample images: tests/diffusion/quantization/
+# test_quantization_fp8.py saves these into the repo root for manual
+# inspection; they are byproducts and must never be committed.
+test_*_fp8.png