Add v4.2 wishlist with GPU decode idea

Blue sky document for capturing future feature ideas.

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

WISHLIST-4.2.md

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+# Stegasoo v4.2 Wishlist
+
+Blue sky ideas for future development. No timeline - just capturing thoughts.
+
+---
+
+## Performance
+
+### GPU-Accelerated DCT Encoding/Decoding
+- **Idea**: Leverage GPU for JPEG DCT coefficient manipulation
+- **Potential Approaches**:
+  - OpenCL/CUDA for parallel DCT operations
+  - Raspberry Pi VideoCore IV/VI GPU compute
+  - WebGPU for browser-based acceleration
+- **Challenges**:
+  - jpegio library is CPU-bound (C extension)
+  - Would need custom DCT implementation
+  - Memory transfer overhead may negate gains for small images
+- **Research**:
+  - libjpeg-turbo uses SIMD but not GPU
+  - nvJPEG (NVIDIA) does GPU-accelerated JPEG
+  - Could potentially use GPU for the embedding math, not JPEG decode
+
+---
+
+## Features
+
+(Add ideas here)
+
+---
+
+## Infrastructure
+
+(Add ideas here)
+
+---
+
+## Notes
+
+- This is a living document - add ideas anytime
+- Not all ideas will be implemented
+- Feasibility research needed before committing to roadmap