CNN v2: Fix weight buffer offset bug

Root cause: Binary format is [header:20B][layer_info:20B×N][weights].
Both cnn_test and CNNv2Effect uploaded entire file to weights_buffer,
but shader reads weights_buffer[0] expecting first weight, not header.

Fix: Skip header + layer_info when uploading to GPU buffer.
- cnn_test.cc: Calculate weights_offset, upload only weights section
- cnn_v2_effect.cc: Same fix for runtime effect

Before: layer_0 output showed [R, uv_x, uv_y, black] (wrong channels)
After: layer_0 output shows [R, G, B, D] (correct identity mapping)

Tests: 34/36 passing (2 unrelated failures)

0 files changed, 0 insertions, 0 deletions