Add CNN v2 WebGPU testing tool

Implements single-file HTML tool for rapid CNN weight validation:

Features:
- Drag-drop PNG images (whole window) and .bin weights
- Real-time WebGPU compute pipeline (static features + N layers)
- Data-driven execution (reads layer count from binary)
- View modes: CNN output / Original / Diff (×10)
- Blend slider (0.0-1.0) for effect strength
- Console log with timestamps
- Keyboard shortcuts: SPACE (original), D (diff)

Architecture:
- Embedded WGSL shaders (static + compute + display)
- Binary parser for .bin format (header + layer info + f16 weights)
- Persistent textures for view mode switching
- Absolute weight offset calculation (header + layer info skip)

Implementation notes:
- Weight offsets in binary are relative to weights section
- JavaScript precalculates absolute offsets: headerOffsetU32 * 2 + offset
- Matches C++ shader behavior (simple get_weight without offset param)
- Ping-pong textures for multi-layer processing

TODO:
- Side panel: .bin metadata, weight statistics, validation
- Layer inspection: R/G/B/A plane split, intermediate outputs
- Activation heatmaps for debugging

Files:
- tools/cnn_v2_test/index.html (24 KB, 730 lines)
- tools/cnn_v2_test/README.md (usage guide, troubleshooting)

handoff(Claude): CNN v2 HTML testing tool complete, documented TODOs for future enhancements

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