test: Add GPU effects test infrastructure (Phase 1 Foundation)

Creates shared testing utilities for headless GPU effect testing.
Enables testing visual effects without windows (CI-friendly).

New Test Infrastructure (8 files):
- webgpu_test_fixture.{h,cc}: Shared WebGPU initialization
  * Handles Win32 (old API) vs Native (new callback info structs)
  * Graceful skip if GPU unavailable
  * Eliminates 100+ lines of boilerplate per test

- offscreen_render_target.{h,cc}: Headless rendering ("frame sink")
  * Creates offscreen WGPUTexture for rendering without windows
  * Pixel readback via wgpuBufferMapAsync for validation
  * 262,144 byte framebuffer (256x256 BGRA8)

- effect_test_helpers.{h,cc}: Reusable validation utilities
  * has_rendered_content(): Detects non-black pixels
  * all_pixels_match_color(): Color matching with tolerance
  * hash_pixels(): Deterministic output verification (FNV-1a)

- test_effect_base.cc: Comprehensive test suite (7 tests, all passing)
  * WebGPU fixture lifecycle
  * Offscreen rendering and pixel readback
  * Effect construction and initialization
  * Sequence add_effect and activation logic
  * Pixel validation helpers

Coverage Impact:
- GPU test infrastructure: 0% → Foundation ready for Phase 2
- Next: Individual effect tests (FlashEffect, GaussianBlur, etc.)

Size Impact: ZERO
- All test code wrapped in #if !defined(STRIP_ALL)
- Test executables separate from demo64k
- No impact on final binary (verified with guards)

Test Output:
  ✓ 7/7 tests passing
  ✓ WebGPU initialization (adapter + device)
  ✓ Offscreen render target creation
  ✓ Pixel readback (262,144 bytes)
  ✓ Effect initialization via Sequence
  ✓ Sequence activation logic
  ✓ Pixel validation helpers

Technical Details:
- Uses WGPUTexelCopyTextureInfo/BufferInfo (not deprecated ImageCopy*)
- Handles WGPURequestAdapterCallbackInfo (native) vs old API (Win32)
- Polls wgpuInstanceProcessEvents for async operations
- MapAsync uses WGPUMapMode_Read for pixel readback

Analysis Document:
- GPU_EFFECTS_TEST_ANALYSIS.md: Full roadmap (Phases 1-4, 44 hours)
- Phase 1 complete, Phase 2 ready (individual effect tests)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

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