fix(gpu): stop gpu_tests hanging under the parallel test runner

Every test created its own wgpu instance and device (with adapter-maximum
limits) concurrently, which could wedge the driver and hang the suite
indefinitely. Tests now hold a process-wide lock while they own a device, and
GpuAccelerator readback waits are bounded at 30s so a stuck driver surfaces as
GpuError::BufferMap instead of blocking forever.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

crates/clawhdf5-gpu/src/accelerator.rs

@@ -6,6 +6,9 @@ use crate::shaders;
 use bytemuck::Pod;
 use wgpu::util::DeviceExt;
 
+/// Upper bound on a single GPU→CPU readback wait.
+const READBACK_TIMEOUT: std::time::Duration = std::time::Duration::from_secs(30);
+
 /// GPU-accelerated vector search engine.
 ///
 /// Upload vectors once, then run many searches against them.
@@ -1033,10 +1036,12 @@ impl GpuAccelerator {
         slice.map_async(wgpu::MapMode::Read, move |result| {
             let _ = tx.send(result);
         });
+        // Bounded wait: a wedged driver must surface as an error, not hang
+        // the caller forever.
         self.device
             .poll(wgpu::PollType::Wait {
                 submission_index: None,
-                timeout: None,
+                timeout: Some(READBACK_TIMEOUT),
             })
             .map_err(|e| GpuError::BufferMap(format!("device poll failed: {e}")))?;
         rx.recv()