Add bounds checking to KVCache.advance() method

- Added validation to prevent cache overflow beyond max_seq_len
- Raises ValueError if cache position would exceed maximum sequence length
- This helps catch potential bugs early during inference

nanochat/engine.py

@@ -115,7 +115,11 @@ class KVCache:
 
     def advance(self, num_tokens):
         """Advance the cache position by num_tokens."""
-        self.cache_seqlens += num_tokens
+        # Validate that we don't exceed max sequence length
+        new_seqlens = self.cache_seqlens + num_tokens
+        if torch.any(new_seqlens > self.max_seq_len):
+            raise ValueError(f"Cache overflow: attempted to advance beyond max_seq_len={self.max_seq_len}")
+        self.cache_seqlens.copy_(new_seqlens)
 
     def prefill(self, other):
         """