CNN v2: Add mip-level support to runtime effect

Binary format v2 includes mip_level in header (20 bytes, was 16).
Effect reads mip_level and passes to static features shader via uniform.
Shader samples from correct mip texture based on mip_level.

Changes:
- export_cnn_v2_weights.py: Header v2 with mip_level field
- cnn_v2_effect.h: Add StaticFeatureParams, mip_level member, params buffer
- cnn_v2_effect.cc: Read mip_level from weights, create/bind params buffer, update per-frame
- cnn_v2_static.wgsl: Accept params uniform, sample from selected mip level

Binary format v2:
- Header: 20 bytes (magic, version=2, num_layers, total_weights, mip_level)
- Backward compatible: v1 weights load with mip_level=0

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

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