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, 130 insertions, 0 deletions

diff --git a/src/tests/audio/test_audio_backend.cc b/src/tests/audio/test_audio_backend.cc
new file mode 100644
index 0000000..6a748aa
--- /dev/null
+++ b/src/tests/audio/test_audio_backend.cc
@@ -0,0 +1,130 @@
+// This file is part of the 64k demo project.
+// It tests the audio backend abstraction layer.
+// Verifies backend injection and event hooks work correctly.
+
+#include "audio/audio.h"
+#include "audio/audio_backend.h"
+#include "audio/synth.h"
+#include <assert.h>
+#include <stdio.h>
+#include <vector>
+
+#if !defined(STRIP_ALL)
+
+// Simple test backend that records events
+class TestBackend : public AudioBackend {
+ public:
+  struct Event {
+    float timestamp;
+    int spectrogram_id;
+    float volume;
+    float pan;
+  };
+
+  std::vector<Event> events;
+  bool init_called = false;
+  bool start_called = false;
+  bool shutdown_called = false;
+
+  void init() override {
+    init_called = true;
+  }
+
+  void start() override {
+    start_called = true;
+  }
+
+  void shutdown() override {
+    shutdown_called = true;
+  }
+
+  float get_realtime_peak() override {
+    // Test backend: return synthetic peak
+    return 0.5f;
+  }
+
+  void on_voice_triggered(float timestamp, int spectrogram_id, float volume,
+                          float pan) override {
+    events.push_back({timestamp, spectrogram_id, volume, pan});
+  }
+};
+
+void test_backend_injection() {
+  TestBackend backend;
+
+  // Inject test backend before audio_init
+  audio_set_backend(&backend);
+
+  audio_init();
+  assert(backend.init_called);
+
+  audio_start();
+  assert(backend.start_called);
+
+  audio_shutdown();
+  assert(backend.shutdown_called);
+
+  printf("Backend injection test PASSED\n");
+}
+
+void test_event_recording() {
+  TestBackend backend;
+  audio_set_backend(&backend);
+
+  synth_init();
+
+  // Create a dummy spectrogram
+  float data[DCT_SIZE * 2] = {0};
+  Spectrogram spec = {data, data, 2};
+  int id = synth_register_spectrogram(&spec);
+
+  // Trigger a voice
+  synth_trigger_voice(id, 0.8f, -0.5f);
+
+  // Render some frames to advance time
+  float output[1024] = {0};
+  synth_render(output, 256); // ~0.008 sec at 32kHz
+
+  // Verify event was recorded
+  assert(backend.events.size() == 1);
+  assert(backend.events[0].spectrogram_id == id);
+  assert(backend.events[0].volume == 0.8f);
+  assert(backend.events[0].pan == -0.5f);
+  assert(backend.events[0].timestamp == 0.0f); // Triggered before any render
+
+  // Trigger another voice after rendering
+  synth_trigger_voice(id, 1.0f, 0.0f);
+
+  assert(backend.events.size() == 2);
+  assert(backend.events[1].timestamp > 0.0f); // Should be > 0 now
+
+  printf("Event recording test PASSED\n");
+}
+
+void test_default_backend() {
+  // Reset backend to nullptr to test default
+  audio_set_backend(nullptr);
+
+  // This should use MiniaudioBackend by default
+  audio_init();
+  audio_start();
+  audio_shutdown();
+
+  printf("Default backend test PASSED\n");
+}
+
+#endif /* !defined(STRIP_ALL) */
+
+int main() {
+#if !defined(STRIP_ALL)
+  printf("Running Audio Backend tests...\n");
+  test_backend_injection();
+  test_event_recording();
+  test_default_backend();
+  printf("All Audio Backend tests PASSED\n");
+  return 0;
+#else
+  printf("Audio Backend tests skipped (STRIP_ALL enabled)\n");
+  return 0;
+#endif /* !defined(STRIP_ALL) */
+}