build: Skip regenerating kernels when source unchanged

  Prevents CI from regenerating non-reproducible .a files unnecessarily.
  Only regenerates when .cu source changes or artifacts are missing.
  Delete .md5 files to force regeneration after changing CUDA_ARCHS.

Author: oandreeva-nv

lib/kvbm-kernels/README.md

@@ -247,6 +247,19 @@ cargo build --release
 # Commit the updated cuda/prebuilt/tensor_kernels.{fatbin,md5}
 ```
 
+**Important:** If you change `CUDA_ARCHS` or update your nvcc version, you need to
+force regeneration by deleting the checksums:
+
+```bash
+# Force regeneration after changing CUDA_ARCHS or nvcc version
+rm cuda/prebuilt/*.md5
+cargo build --release
+# Commit the updated files
+```
+
+The build system only checks if the `.cu` source has changed, not build configuration.
+This prevents CI from regenerating non-reproducible `.a` files unnecessarily.
+
 ---
 
 ### Python Bindings & Tests

lib/kvbm-kernels/build.rs

@@ -246,6 +246,30 @@ fn generate_prebuilt_artifacts(cu_path: &Path, arch_flags: &[String], out_dir: &
     let lib_path = prebuilt_dir.join(format!("lib{}.a", kernel_name));
     let md5_path = prebuilt_dir.join(format!("{}.md5", kernel_name));
 
+    // Skip regeneration if valid prebuilt artifacts already exist
+    // (avoids modifying source tree with non-reproducible .a files in CI)
+    //
+    // LIMITATION: This only checks if the .cu source has changed, not build configuration
+    // (CUDA_ARCHS, nvcc version, compiler flags). If you change CUDA_ARCHS or update nvcc,
+    // manually delete the .md5 files to force regeneration:
+    //   rm lib/kvbm-kernels/cuda/prebuilt/*.md5
+    if fatbin_path.exists() && lib_path.exists() && md5_path.exists() {
+        // Validate that existing artifacts match current source
+        if let Ok(stored_hashes) = fs::read_to_string(&md5_path) {
+            let hashes: Vec<&str> = stored_hashes.lines().collect();
+            if hashes.len() >= 2 {
+                let current_cu_hash = compute_file_hash(cu_path);
+                if current_cu_hash == hashes[0] {
+                    println!(
+                        "cargo:warning=Skipping regeneration of {} (valid prebuilt artifacts exist)",
+                        kernel_name
+                    );
+                    return;
+                }
+            }
+        }
+    }
+
     // Generate .fatbin using nvcc
     let temp_fatbin = Path::new(out_dir).join(format!("{}.fatbin", kernel_name));
     // Generate .o (object file) for static library