fix: leak HDF5 library handle to prevent cleanup issues

Root cause: When the OnceLock<Library> was dropped at process exit,
dlclose() was called on the HDF5 library. This triggered HDF5's
internal cleanup routines which caused 'infinite loop closing library'
and SIGSEGV on Linux, especially during parallel test execution.

Solution: Use Box::leak() to intentionally leak the library handle.
This prevents dlclose() from being called, keeping the HDF5 library
loaded until process termination. This is safe because:
1. We only load the library once per process
2. The OS will reclaim all memory on process exit
3. This is a common pattern for libraries with problematic cleanup

Also reverts the CI workaround (--test-threads=1) since the root
cause is now fixed.

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

Author: shinaoka

hdf5/src/sys/runtime.rs

@@ -832,7 +832,7 @@ pub const HDF5_VERSION: Version = Version { major: 1, minor: 14, micro: 0 };
 // Library management
 // =============================================================================
 
-static LIBRARY: OnceLock<Library> = OnceLock::new();
+static LIBRARY: OnceLock<&'static Library> = OnceLock::new();
 static LIBRARY_PATH: OnceLock<String> = OnceLock::new();
 static HDF5_RUNTIME_VERSION: OnceLock<Version> = OnceLock::new();
 
@@ -841,7 +841,7 @@ pub static LOCK: ReentrantMutex<()> = ReentrantMutex::new(());
 
 /// Get the library handle
 fn get_library() -> &'static Library {
-    LIBRARY.get().expect("HDF5 library not initialized. Call hdf5::sys::init() first.")
+    *LIBRARY.get().expect("HDF5 library not initialized. Call hdf5::sys::init() first.")
 }
 
 /// Initialize the HDF5 library by loading it from the specified path.
@@ -872,6 +872,13 @@ pub fn init(path: Option<&str>) -> Result<(), String> {
     let library = unsafe { Library::new(&lib_path) }
         .map_err(|e| format!("Failed to load HDF5 library from {}: {}", lib_path, e))?;
 
+    // Leak the library handle to prevent dlclose() on exit.
+    // HDF5 has problematic cleanup routines that can cause "infinite loop closing library"
+    // and SIGSEGV if the library is unloaded while HDF5 internal state still exists.
+    // This is safe because we only load the library once per process and it should
+    // remain loaded until process exit.
+    let library = Box::leak(Box::new(library));
+
     LIBRARY.set(library).map_err(|_| "Library already initialized".to_string())?;
     LIBRARY_PATH.set(lib_path).map_err(|_| "Library path already set".to_string())?;