✨ feat: add DensityObserver for XAttention sparse attention density tracking

- Add DensityObserver class to track per-layer density statistics
- Integrate DensityObserver into compute_prefill for GPU-only mode
- Fix stride parameter not being passed to xattn_estimate
- Add density statistics output to test_ruler.py for XATTN_BSA
- Add comprehensive density benchmark documentation

Key changes:
- nanovllm/utils/density_observer.py: New Observer for density tracking
- xattn_bsa.py: Add stride param to xattn_estimate, integrate DensityObserver
- test_ruler.py: Enable DensityObserver and print summary for XATTN_BSA
- docs/xattn_density_benchmark.md: Benchmark results for 4K-32K contexts

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

tests/test_ruler.py

@@ -41,6 +41,7 @@ from pathlib import Path
 from typing import List, Dict, Tuple, Optional
 
 from nanovllm import LLM, SamplingParams
+from nanovllm.utils.density_observer import DensityObserver
 
 
 # ============================================================
@@ -381,6 +382,13 @@ def run_ruler_benchmark(
         print(f"Fresh LLM mode: {fresh_llm}")
         print(f"{'='*60}")
 
+    # Enable DensityObserver for XAttention BSA
+    if sparse_policy and sparse_policy.upper() == "XATTN_BSA":
+        DensityObserver.enable()
+        DensityObserver.complete_reset()
+        if not json_output:
+            print("[DensityObserver] Enabled for XAttention BSA")
+
     # LLM initialization kwargs
     llm_kwargs = {
         "max_model_len": max_model_len,
@@ -471,6 +479,14 @@ def run_ruler_benchmark(
         print(f"{'-'*54}")
         print(f"{'TOTAL':<20} {total_correct}/{total_samples:<7} {overall_accuracy*100:>6.1f}%      {avg_score:.3f}")
         print(f"\nTime: {total_time:.1f}s")
+
+        # Print DensityObserver summary if enabled
+        if sparse_policy and sparse_policy.upper() == "XATTN_BSA" and DensityObserver.is_enabled():
+            print(f"\n{'='*60}")
+            print("Density Statistics (XAttention BSA)")
+            print(f"{'='*60}")
+            DensityObserver.print_summary()
+
         print(f"{'='*60}\n")
 
     results = {