CNN v2 web tool: Multiple fixes for feature parity with cnn_test

Changes:
- Static shader: Point sampler (nearest filter) instead of linear
- Mip handling: Use textureSampleLevel with point sampler (fixes coordinate scaling)
- Save PNG: GPU readback via staging buffer (WebGPU canvas lacks toBlob support)
- Depth binding: Use input texture as depth (matches C++ simplification)
- Header offset: Version-aware calculation (v1=4, v2=5 u32)

Known issue: Output still differs from cnn_test (color tones). Root cause TBD.

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

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