test(gpu): Add automatic validation for effect test coverage

Problem: When new effects are added to demo_effects.h, developers might
forget to update test_demo_effects.cc, leading to untested code.

Solution: Added compile-time constants and runtime assertions to enforce
test coverage:

1. Added EXPECTED_POST_PROCESS_COUNT = 8
2. Added EXPECTED_SCENE_COUNT = 6
3. Runtime validation in each test function
4. Fails with clear error message if counts don't match

Error message when validation fails:
  ✗ COVERAGE ERROR: Expected N effects, but only tested M!
  ✗ Did you add a new effect without updating the test?
  ✗ Update EXPECTED_*_COUNT in test_demo_effects.cc

Updated CONTRIBUTING.md with mandatory test update requirement:
- Added step 3 to "Adding a New Visual Effect" workflow
- Clear instructions on updating effect counts
- Verification command examples

This ensures no effect can be added without corresponding test coverage.

Tested validation by intentionally breaking count - error caught correctly.

1 files changed, 12 insertions, 1 deletions

diff --git a/doc/CONTRIBUTING.md b/doc/CONTRIBUTING.md
index 4b85e9f..7d206a5 100644
--- a/doc/CONTRIBUTING.md
+++ b/doc/CONTRIBUTING.md
@@ -255,7 +255,18 @@ Make sure everything is reflected in clang-format.
     -   Implement `compute()` if you need GPU-side physics or state updates.
     -   Implement `render()` to record WebGPU draw commands.
 2.  **Register**: Add an `EFFECT` entry to `assets/demo.seq` specifying the class name, start/end times, and any constructor arguments.
-3.  **Verify**: Build with `DEMO_ALL_OPTIONS=ON` and use `--seek` to test your effect at its specific timestamp.
+3.  **Update Tests** (REQUIRED): Add your effect to `src/tests/test_demo_effects.cc`:
+    -   Add effect to the appropriate test list (`test_post_process_effects()` or `test_scene_effects()`)
+    -   Increment `EXPECTED_POST_PROCESS_COUNT` or `EXPECTED_SCENE_COUNT` at the top of the file
+    -   If your effect requires `Renderer3D` with full shader setup, add it to the `requires_3d` check
+    -   The test will fail with a clear error message if you forget this step
+4.  **Verify**: Build with `DEMO_ALL_OPTIONS=ON` and run tests:
+    ```bash
+    cmake -S . -B build -DDEMO_BUILD_TESTS=ON
+    cmake --build build --target test_demo_effects
+    cd build && ./test_demo_effects
+    ```
+    Then test your effect at its specific timestamp with `--seek`.
 
 ### Audio Subsystem Initialization