refactor: Reorganize tests into subsystem subdirectories

Restructured test suite for better organization and targeted testing:

**Structure:**
- src/tests/audio/    - 15 audio system tests
- src/tests/gpu/      - 12 GPU/shader tests
- src/tests/3d/       - 6 3D rendering tests
- src/tests/assets/   - 2 asset system tests
- src/tests/util/     - 3 utility tests
- src/tests/common/   - 3 shared test helpers
- src/tests/scripts/  - 2 bash test scripts (moved conceptually, not physically)

**CMake changes:**
- Updated add_demo_test macro to accept LABEL parameter
- Applied CTest labels to all 36 tests for subsystem filtering
- Updated all test file paths in CMakeLists.txt
- Fixed common helper paths (webgpu_test_fixture, etc.)
- Added custom targets for subsystem testing:
  - run_audio_tests, run_gpu_tests, run_3d_tests
  - run_assets_tests, run_util_tests, run_all_tests

**Include path updates:**
- Fixed relative includes in GPU tests to reference ../common/

**Documentation:**
- Updated doc/HOWTO.md with subsystem test commands
- Updated doc/CONTRIBUTING.md with new test organization
- Updated scripts/check_all.sh to reflect new structure

**Verification:**
- All 36 tests passing (100%)
- ctest -L <subsystem> filters work correctly
- make run_<subsystem>_tests targets functional
- scripts/check_all.sh passes

Backward compatible: make test and ctest continue to work unchanged.

handoff(Gemini): Test reorganization complete. 36/36 tests passing.

1 files changed, 141 insertions, 0 deletions

diff --git a/src/tests/common/webgpu_test_fixture.cc b/src/tests/common/webgpu_test_fixture.cc
new file mode 100644
index 0000000..afb7ce3
--- /dev/null
+++ b/src/tests/common/webgpu_test_fixture.cc
@@ -0,0 +1,141 @@
+// This file is part of the 64k demo project.
+// It implements shared WebGPU initialization for GPU tests.
+// Provides graceful fallback if GPU unavailable.
+
+#include "webgpu_test_fixture.h"
+#include <cstdio>
+#include <cstdlib>
+
+WebGPUTestFixture::WebGPUTestFixture() {
+}
+
+WebGPUTestFixture::~WebGPUTestFixture() {
+  shutdown();
+}
+
+bool WebGPUTestFixture::init() {
+  // Create instance
+  const WGPUInstanceDescriptor instance_desc = {};
+  instance_ = wgpuCreateInstance(&instance_desc);
+  if (!instance_) {
+    fprintf(stderr,
+            "WebGPU not available (wgpuCreateInstance failed) - skipping GPU "
+            "test\n");
+    return false;
+  }
+
+  // Request adapter (API differs between Win32 and native)
+  WGPUAdapter adapter = nullptr;
+  const WGPURequestAdapterOptions adapter_opts = {
+      .compatibleSurface = nullptr,
+      .powerPreference = WGPUPowerPreference_HighPerformance,
+  };
+
+#if defined(DEMO_CROSS_COMPILE_WIN32)
+  // Win32: Old callback API (function pointer + userdata)
+  auto on_adapter = [](WGPURequestAdapterStatus status, WGPUAdapter a,
+                       const char* message, void* userdata) {
+    if (status == WGPURequestAdapterStatus_Success) {
+      *(WGPUAdapter*)userdata = a;
+    } else if (message) {
+      fprintf(stderr, "Adapter request failed: %s\n", message);
+    }
+  };
+  wgpuInstanceRequestAdapter(instance_, &adapter_opts, on_adapter, &adapter);
+#else
+  // Native: New callback info API
+  auto on_adapter = [](WGPURequestAdapterStatus status, WGPUAdapter a,
+                       WGPUStringView message, void* userdata, void* user2) {
+    (void)user2;
+    (void)message;
+    if (status == WGPURequestAdapterStatus_Success) {
+      *(WGPUAdapter*)userdata = a;
+    }
+  };
+  WGPURequestAdapterCallbackInfo adapter_cb = {};
+  adapter_cb.mode = WGPUCallbackMode_WaitAnyOnly;
+  adapter_cb.callback = on_adapter;
+  adapter_cb.userdata1 = &adapter;
+  wgpuInstanceRequestAdapter(instance_, &adapter_opts, adapter_cb);
+#endif
+
+  // Wait for adapter callback
+  for (int i = 0; i < 100 && !adapter; ++i) {
+    wgpuInstanceProcessEvents(instance_);
+  }
+
+  if (!adapter) {
+    fprintf(stderr, "No WebGPU adapter available - skipping GPU test\n");
+    shutdown();
+    return false;
+  }
+
+  adapter_ = adapter;
+
+  // Request device (API differs between Win32 and native)
+  WGPUDevice device = nullptr;
+  const WGPUDeviceDescriptor device_desc = {};
+
+#if defined(DEMO_CROSS_COMPILE_WIN32)
+  // Win32: Old callback API
+  auto on_device = [](WGPURequestDeviceStatus status, WGPUDevice d,
+                      const char* message, void* userdata) {
+    if (status == WGPURequestDeviceStatus_Success) {
+      *(WGPUDevice*)userdata = d;
+    } else if (message) {
+      fprintf(stderr, "Device request failed: %s\n", message);
+    }
+  };
+  wgpuAdapterRequestDevice(adapter_, &device_desc, on_device, &device);
+#else
+  // Native: New callback info API
+  auto on_device = [](WGPURequestDeviceStatus status, WGPUDevice d,
+                      WGPUStringView message, void* userdata, void* user2) {
+    (void)user2;
+    (void)message;
+    if (status == WGPURequestDeviceStatus_Success) {
+      *(WGPUDevice*)userdata = d;
+    }
+  };
+  WGPURequestDeviceCallbackInfo device_cb = {};
+  device_cb.mode = WGPUCallbackMode_WaitAnyOnly;
+  device_cb.callback = on_device;
+  device_cb.userdata1 = &device;
+  wgpuAdapterRequestDevice(adapter_, &device_desc, device_cb);
+#endif
+
+  // Wait for device callback
+  for (int i = 0; i < 100 && !device; ++i) {
+    wgpuInstanceProcessEvents(instance_);
+  }
+
+  if (!device) {
+    fprintf(stderr, "Failed to create WebGPU device - skipping GPU test\n");
+    shutdown();
+    return false;
+  }
+
+  device_ = device;
+  queue_ = wgpuDeviceGetQueue(device_);
+
+  return true;
+}
+
+void WebGPUTestFixture::shutdown() {
+  if (queue_) {
+    wgpuQueueRelease(queue_);
+    queue_ = nullptr;
+  }
+  if (device_) {
+    wgpuDeviceRelease(device_);
+    device_ = nullptr;
+  }
+  if (adapter_) {
+    wgpuAdapterRelease(adapter_);
+    adapter_ = nullptr;
+  }
+  if (instance_) {
+    wgpuInstanceRelease(instance_);
+    instance_ = nullptr;
+  }
+}