CNN v2 Web Tool: Add layer/weight visualization with debug infrastructure

Features:
- Right sidebar with Layer Visualization (top) and Weights Info (collapsible, bottom)
- Activations mode: 4-channel grayscale views per layer (Static L0 + CNN layers)
- Weights mode: Kernel visualization with 2D canvas rendering
- Mode tabs to switch between activation and weight inspection
- Per-layer texture storage (separate from ping-pong compute buffers)
- Debug shader modes (UV gradient, raw packed data, unpacked f16)
- Comprehensive logging for diagnostics

Architecture:
- Persistent layerTextures[] for visualization (one per layer)
- Separate computeTextures[] for CNN ping-pong
- copyTextureToTexture after each layer pass
- Canvas recreation on mode switch (2D vs WebGPU context)
- Weight parsing with f16 unpacking and min/max calculation

Known Issues:
- Layer activations show black (texture data empty despite copies)
- Weight kernels not displaying (2D canvas renders not visible)
- Debug mode 10 (UV gradient) works, confirming texture access OK
- Root cause: likely GPU command ordering or texture usage flags

Documentation:
- Added doc/CNN_V2_WEB_TOOL.md with full status, architecture, debug steps
- Detailed issue tracking with investigation notes and next steps

Status: Infrastructure complete, debugging data flow issues.

handoff(Claude): Layer viz black due to empty textures despite copyTextureToTexture.
Weight viz black despite correct canvas setup. Both issues need GPU pipeline audit.

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