🔧 feat: add density statistics tracking to sparse policies

Add statistics tracking to compare block selection between policies:
- XAttentionBSAPolicy: track available/selected blocks per chunk
- FullAttentionPolicy: track total blocks (always 100% density)
- Add reset_stats(), get_density_stats(), print_density_stats() methods
- Use logger.debug for per-chunk density logging

Results on 32K niah_single_1:
- Full: 100% density across all chunks
- XAttn BSA: 90% -> 73% density (saves ~25-30% blocks in later chunks)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

nanovllm/kvcache/sparse/full_policy.py

@@ -37,6 +37,11 @@ class FullAttentionPolicy(SparsePolicy):
     supports_prefill = True
     supports_decode = True
 
+    def __init__(self):
+        """Initialize with statistics tracking."""
+        self._stats_total_blocks = 0
+        self._stats_num_chunks = 0
+
     def select_blocks(
         self,
         available_blocks: List[int],
@@ -44,8 +49,33 @@ class FullAttentionPolicy(SparsePolicy):
         ctx: PolicyContext,
     ) -> List[int]:
         """Return all blocks - no sparsity."""
+        # Update statistics (only for layer 0 to avoid overcounting)
+        if ctx.layer_id == 0 and available_blocks:
+            self._stats_total_blocks += len(available_blocks)
+            self._stats_num_chunks += 1
+            logger.debug(f"[Full] chunk={ctx.query_chunk_idx}, blocks={len(available_blocks)}, density=100.0%")
         return available_blocks
 
+    def reset_stats(self) -> None:
+        """Reset density statistics."""
+        self._stats_total_blocks = 0
+        self._stats_num_chunks = 0
+
+    def get_density_stats(self) -> dict:
+        """Get density statistics."""
+        return {
+            "total_available_blocks": self._stats_total_blocks,
+            "total_selected_blocks": self._stats_total_blocks,  # Full = all selected
+            "num_chunks": self._stats_num_chunks,
+            "overall_density": 1.0,  # Always 100%
+        }
+
+    def print_density_stats(self) -> None:
+        """Print density statistics summary."""
+        stats = self.get_density_stats()
+        logger.info(f"[Full Policy] Density Stats: chunks={stats['num_chunks']}, "
+                   f"blocks={stats['total_available_blocks']}, density=100.0%")
+
     def compute_chunked_prefill(
         self,
         q: torch.Tensor,