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, 184 insertions, 0 deletions

diff --git a/src/tests/assets/test_assets.cc b/src/tests/assets/test_assets.cc
new file mode 100644
index 0000000..2ee18d6
--- /dev/null
+++ b/src/tests/assets/test_assets.cc
@@ -0,0 +1,184 @@
+// This file is part of the 64k demo project.
+// It tests the asset manager's ability to retrieve packed data.
+// Verifies data integrity and size reporting.
+
+#if defined(USE_TEST_ASSETS)
+#include "test_assets.h"
+#else
+#include "generated/assets.h"
+#endif /* defined(USE_TEST_ASSETS) */
+
+#include "util/asset_manager_utils.h"
+
+#include <assert.h>
+#include <stdio.h>
+#include <string.h>
+
+int main() {
+  printf("Running AssetManager test...\n");
+
+  size_t size = 0;
+  const uint8_t* data1 = GetAsset(AssetId::ASSET_TEST_ASSET_1, &size);
+
+  assert(data1 != nullptr);
+  assert(size > 0);
+
+  const char* expected_prefix = "This is a test asset file.";
+  if (strncmp((const char*)data1, expected_prefix, strlen(expected_prefix)) ==
+      0) {
+    printf("Asset content verification: SUCCESS\n");
+  } else {
+    printf("Asset content verification: FAILED\n");
+    printf("Got: %.*s\n", (int)size, (const char*)data1);
+    return 1;
+  }
+
+  // Test caching: request the same asset again and verify pointer is identical
+  size_t size2 = 0;
+  const uint8_t* data2 = GetAsset(AssetId::ASSET_TEST_ASSET_1, &size2);
+  assert(data2 != nullptr);
+  assert(size2 == size);
+  assert(data1 == data2); // Pointers should be the same for cached static asset
+  printf("Asset caching test: SUCCESS\n");
+
+  // Test ASSET_LAST_ID - should not return a valid asset
+  size_t last_id_size = 0;
+  const uint8_t* last_id_data = GetAsset(AssetId::ASSET_LAST_ID, &last_id_size);
+  assert(last_id_data == nullptr);
+  assert(last_id_size == 0);
+  printf("ASSET_LAST_ID test: SUCCESS\n");
+
+  printf("Asset size: %zu bytes\n", size);
+
+  // Test procedural asset
+  printf("\nRunning Procedural Asset test...\n");
+  size_t proc_size = 0;
+  const uint8_t* proc_data_1 =
+      GetAsset(AssetId::ASSET_PROC_NOISE_256, &proc_size);
+  assert(proc_data_1 != nullptr);
+  // Expect 256x256 RGBA8 + 8 byte header
+  assert(proc_size == 256 * 256 * 4 + 8);
+
+  // Verify first few bytes of DATA (skip header)
+  // Header is 8 bytes
+  const uint8_t* pixel_data_1 = proc_data_1 + 8;
+  bool non_zero_data = false;
+  for (size_t i = 0; i < 16; ++i) { // Check first 16 bytes of pixels
+    if (pixel_data_1[i] != 0) {
+      non_zero_data = true;
+      break;
+    }
+  }
+  assert(non_zero_data);
+  printf("Procedural asset content verification: SUCCESS\n");
+
+  // Test DropAsset for procedural asset and re-generation
+  DropAsset(AssetId::ASSET_PROC_NOISE_256, proc_data_1);
+  // After dropping, GetAsset should generate new data
+  const uint8_t* proc_data_2 =
+      GetAsset(AssetId::ASSET_PROC_NOISE_256, &proc_size);
+  assert(proc_data_2 != nullptr);
+  // assert(proc_data_1 != proc_data_2); // Removed: Allocator might reuse the
+  // same address
+
+  // Verify content again to ensure it was re-generated correctly
+  non_zero_data = false;
+  const uint8_t* pixel_data_2 = proc_data_2 + 8;
+  for (size_t i = 0; i < 16; ++i) {
+    if (pixel_data_2[i] != 0) {
+      non_zero_data = true;
+      break;
+    }
+  }
+  assert(non_zero_data);
+  printf("Procedural asset DropAsset and re-generation test: SUCCESS\n");
+
+  // Test Texture Asset (TGA loading)
+  printf("\nRunning Texture Asset test...\n");
+  TextureAsset tex = GetTextureAsset(AssetId::ASSET_TEST_IMAGE);
+  assert(tex.pixels != nullptr);
+  assert(tex.width == 2);
+  assert(tex.height == 2);
+
+  // Verify pixels (Expected RGBA)
+  // Pixel 0: Red (255, 0, 0, 255)
+  assert(tex.pixels[0] == 255 && tex.pixels[1] == 0 && tex.pixels[2] == 0 &&
+         tex.pixels[3] == 255);
+  // Pixel 1: Green (0, 255, 0, 255)
+  assert(tex.pixels[4] == 0 && tex.pixels[5] == 255 && tex.pixels[6] == 0 &&
+         tex.pixels[7] == 255);
+  // Pixel 2: Blue (0, 0, 255, 255)
+  assert(tex.pixels[8] == 0 && tex.pixels[9] == 0 && tex.pixels[10] == 255 &&
+         tex.pixels[11] == 255);
+  // Pixel 3: White (255, 255, 255, 255)
+  assert(tex.pixels[12] == 255 && tex.pixels[13] == 255 &&
+         tex.pixels[14] == 255 && tex.pixels[15] == 255);
+
+  printf("Texture Asset content verification: SUCCESS\n");
+
+  // Test Unknown Procedural Function
+  printf("\nRunning Unknown Procedural Function test...\n");
+  size_t unknown_size = 0;
+  // This should print an error to stderr: "Error: Unknown procedural
+  // function..."
+  const uint8_t* unknown_data =
+      GetAsset(AssetId::ASSET_PROC_UNKNOWN, &unknown_size);
+  assert(unknown_data == nullptr);
+  assert(unknown_size == 0);
+  printf("Unknown Procedural Function test: SUCCESS\n");
+
+  // Test Failing Procedural Function
+  printf("\nRunning Failing Procedural Function test...\n");
+  size_t fail_size = 0;
+  // This should print an error to stderr: "Error: Procedural generation
+  // failed..."
+  const uint8_t* fail_data = GetAsset(AssetId::ASSET_PROC_FAIL, &fail_size);
+  assert(fail_data == nullptr);
+  assert(fail_size == 0);
+  printf("Failing Procedural Function test: SUCCESS\n");
+
+  // Test Out-of-Bounds ID (beyond ASSET_LAST_ID)
+  // Casting to AssetId to suppress compiler warnings if checking strict enum
+  // types
+  printf("\nRunning Out-of-Bounds ID test...\n");
+  size_t oob_size = 0;
+  const uint8_t* oob_data =
+      GetAsset((AssetId)((int)AssetId::ASSET_LAST_ID + 1), &oob_size);
+  assert(oob_data == nullptr);
+  assert(oob_size == 0);
+  printf("Out-of-Bounds ID test: SUCCESS\n");
+
+  // Test DropAsset edge cases
+  printf("\nRunning DropAsset edge cases test...\n");
+  // Invalid ID
+  DropAsset((AssetId)((int)AssetId::ASSET_LAST_ID + 1), nullptr);
+
+  // Mismatched pointer (should do nothing)
+  // We use proc_data_2 which is valid, but pass a different ID (e.g.
+  // ASSET_TEST_ASSET_1 which is static)
+  DropAsset(AssetId::ASSET_TEST_ASSET_1, proc_data_2);
+  // Verify proc_data_2 is still valid (by checking it's in cache).
+  // Note: GetAsset will just return the cached pointer. If DropAsset worked, it
+  // would have been cleared. But wait, DropAsset clears it from cache. The
+  // correct test for "mismatched pointer" is: pass the correct ID but WRONG
+  // pointer. This ensures we don't clear the cache if the user passes a
+  // stale/wrong pointer.
+
+  // Let's try to drop ASSET_PROC_NOISE_256 with a dummy pointer.
+  uint8_t dummy_ptr;
+  DropAsset(AssetId::ASSET_PROC_NOISE_256, &dummy_ptr);
+  // Check if asset is still in cache (should be, as we didn't drop the real
+  // one) We can't peek into g_asset_cache directly from here (it's static). But
+  // GetAsset should return the SAME pointer as proc_data_2 without
+  // re-generation. If it was dropped, GetAsset would re-generate and likely
+  // return a NEW pointer (new allocation).
+  const uint8_t* proc_data_3 = GetAsset(AssetId::ASSET_PROC_NOISE_256, nullptr);
+  assert(proc_data_3 == proc_data_2);
+  printf("DropAsset edge cases test: SUCCESS\n");
+
+  printf("Procedural Asset test PASSED\n");
+
+  printf("AssetManager test PASSED\n");
+
+  return 0;
+}