Pin gradio==6.16.0 to match Spaces base image (was 6.15.2, build failed with ResolutionImpossible)

requirements.txt

@@ -1,4 +1,4 @@
-gradio==6.15.2
+gradio==6.16.0  # must match the version pinned by the HuggingFace Spaces base image; mismatched pin breaks the Space build
 numpy>=1.24
 torch>=2.2          # the bottom "Mixture of Experts" experiment (lazy-loaded)
 transformers>=4.40  # SpikeWhale config/model + SpikeTokenizer (agents/modmind/)