docs: Add CNN effect documentation and update project status

**New Documentation:**
- `doc/CNN_EFFECT.md` (223 lines): Comprehensive implementation guide
  - Architecture overview (file structure, shader composition)
  - Usage examples (C++ API, timeline integration)
  - Training workflow (planned)
  - Implementation details (convolution signatures, weight storage)
  - Size budget breakdown (~5-8 KB total)
  - Testing and troubleshooting

**Updated Documentation:**
- `doc/CNN.md`: Added implementation status section
  - Completed items (✅ modular shaders, C++ class, tests)
  - Pending items (⏳ training script, multi-layer, quantization)
  - Size impact summary
- `PROJECT_CONTEXT.md`:
  - Added "Effects: CNN post-processing foundation" to Current Status
  - Added `CNN_EFFECT.md` to Technical Reference list

**Summary:**
CNN effect foundation complete with modular WGSL architecture, ready for
training script integration. All tests passing (36/36). ~5-8 KB footprint.

handoff(Claude): Documentation complete for CNN effect implementation

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