feat: add XAttention sparse policy integration

Integrate COMPASS XAttention algorithm into nano-vllm's CPU offload
execution path. Uses FlashAttention with native GQA support for
offload mode.

New files:
- nanovllm/kvcache/sparse/utils.py: find_blocks_chunked() utility
- nanovllm/kvcache/sparse/kernels.py: Triton kernels for XAttention
- nanovllm/kvcache/sparse/xattn.py: XAttentionPolicy implementation

Modified:
- nanovllm/config.py: Add XATTN configuration parameters
- nanovllm/engine/model_runner.py: Support XATTN policy
- nanovllm/kvcache/sparse/__init__.py: Register XAttentionPolicy
- tests/test_ruler.py: Add --sparse-policy parameter

Test results (32k ruler):
- NIAH tasks: 12/12 (100%)
- QA/Recall tasks: 11/15 (73%)
- Overall: 23/27 (85%)

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

nanovllm/kvcache/sparse/__init__.py

@@ -24,6 +24,7 @@ from nanovllm.kvcache.sparse.policy import SparsePolicy, PolicyContext
 from nanovllm.kvcache.sparse.full_policy import FullAttentionPolicy
 from nanovllm.kvcache.sparse.quest import QuestPolicy, QuestConfig, BlockMetadataManager
 from nanovllm.kvcache.sparse.minference import MInferencePolicy
+from nanovllm.kvcache.sparse.xattn import XAttentionPolicy
 
 
 def create_sparse_policy(policy_type: SparsePolicyType, **kwargs) -> SparsePolicy:
@@ -65,6 +66,17 @@ def create_sparse_policy(policy_type: SparsePolicyType, **kwargs) -> SparsePolic
             num_recent_diags=kwargs.get("num_recent_diags", 100),
         )
 
+    elif policy_type == SparsePolicyType.XATTN:
+        return XAttentionPolicy(
+            stride=kwargs.get("stride", 8),
+            threshold=kwargs.get("threshold", 0.9),
+            chunk_size=kwargs.get("chunk_size", 16384),
+            use_triton=kwargs.get("use_triton", True),
+            keep_sink=kwargs.get("keep_sink", False),
+            keep_recent=kwargs.get("keep_recent", False),
+            norm=kwargs.get("norm", 1.0),
+        )
+
     else:
         raise ValueError(f"Unknown policy type: {policy_type}")
 
@@ -78,5 +90,6 @@ __all__ = [
     "QuestConfig",
     "BlockMetadataManager",
     "MInferencePolicy",
+    "XAttentionPolicy",
     "create_sparse_policy",
 ]