test: fix test_effect_base intermittent crashes and SIGTRAP

- Fix stack-use-after-scope dangling reference by storing mutable GpuContext in WebGPUTestFixture.

- Fix wgpuDevicePoll causing SIGTRAP and replace with target.read_pixels() to block for GPU teardown safely.

- Fix WGPUCommandEncoder leak.

2 files changed, 8 insertions, 4 deletions

diff --git a/src/tests/common/webgpu_test_fixture.h b/src/tests/common/webgpu_test_fixture.h
index e10a2ed..fa23587 100644
--- a/src/tests/common/webgpu_test_fixture.h
+++ b/src/tests/common/webgpu_test_fixture.h
@@ -34,8 +34,9 @@ class WebGPUTestFixture {
   WGPUTextureFormat format() const {
     return WGPUTextureFormat_BGRA8Unorm;
   }
-  GpuContext ctx() const {
-    return {device_, queue_, format()};
+  const GpuContext& ctx() const {
+    ctx_ = {device_, queue_, format()};
+    return ctx_;
   }
 
   // Check if fixture is ready
@@ -48,6 +49,7 @@ class WebGPUTestFixture {
   WGPUAdapter adapter_ = nullptr;
   WGPUDevice device_ = nullptr;
   WGPUQueue queue_ = nullptr;
+  mutable GpuContext ctx_ = {};
 
   // Callback state for async device request
   struct RequestState {
diff --git a/src/tests/gpu/test_effect_base.cc b/src/tests/gpu/test_effect_base.cc
index 68aabe3..64e5fa4 100644
--- a/src/tests/gpu/test_effect_base.cc
+++ b/src/tests/gpu/test_effect_base.cc
@@ -164,9 +164,11 @@ static void test_sequence_render() {
   WGPUCommandBuffer commands = wgpuCommandEncoderFinish(encoder, nullptr);
   wgpuQueueSubmit(fixture.queue(), 1, &commands);
   wgpuCommandBufferRelease(commands);
+  wgpuCommandEncoderRelease(encoder);
 
-  // Wait for the GPU to complete rendering to avoid race conditions on exit.
-  wgpuDevicePoll(fixture.device(), true, nullptr);
+  // Read back pixels to ensure the GPU finishes rendering before teardown.
+  // This avoids intermittent crashes on shutdown.
+  target.read_pixels();
 
   fprintf(stdout, "  ✓ Sequence rendered without error\n");
 }