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/engine/model_runner.py

@@ -178,19 +178,34 @@ class ModelRunner:
         # Create KV cache manager using factory
         self.kvcache_manager: KVCacheManager = create_kvcache_manager(config)
 
-        # Create sparse prefill policy for GPU-only path
-        # This is separate from CPU offload sparse policy (which uses select_blocks)
+        # Create sparse prefill policy
+        # This is used for both GPU-only and CPU offload modes when policy supports prefill
         self.sparse_prefill_policy = None
-        if not config.enable_cpu_offload and config.sparse_policy != SparsePolicyType.FULL:
+        if config.sparse_policy != SparsePolicyType.FULL:
             from nanovllm.kvcache.sparse import create_sparse_policy
-            policy = create_sparse_policy(
-                config.sparse_policy,
-                vertical_size=config.minference_vertical_size,
-                slash_size=config.minference_slash_size,
-                adaptive_budget=config.minference_adaptive_budget,
-                num_sink_tokens=config.minference_num_sink_tokens,
-                num_recent_diags=config.minference_num_recent_diags,
-            )
+
+            # Get policy-specific parameters based on type
+            if config.sparse_policy == SparsePolicyType.XATTN:
+                policy_kwargs = {
+                    "stride": config.xattn_stride,
+                    "threshold": config.xattn_threshold,
+                    "chunk_size": config.xattn_chunk_size,
+                    "use_triton": config.xattn_use_triton,
+                    "keep_sink": config.xattn_keep_sink,
+                    "keep_recent": config.xattn_keep_recent,
+                    "norm": config.xattn_norm,
+                }
+            else:  # MINFERENCE or others
+                policy_kwargs = {
+                    "vertical_size": config.minference_vertical_size,
+                    "slash_size": config.minference_slash_size,
+                    "adaptive_budget": config.minference_adaptive_budget,
+                    "num_sink_tokens": config.minference_num_sink_tokens,
+                    "num_recent_diags": config.minference_num_recent_diags,
+                }
+
+            policy = create_sparse_policy(config.sparse_policy, **policy_kwargs)
+
             # Only use if policy supports sparse prefill
             if policy.supports_prefill:
                 self.sparse_prefill_policy = policy