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.

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