Fix CNN v2 static feature channel mapping (p4-p7 → channels 8-11)

Fixed bug in gen_identity_weights.py --p47 mode: static features p4-p7
(uv_x, uv_y, sin20_y, bias) are at input channels 8-11, not 4-7.

Weight tensor layout:
- Channels 0-3: Previous layer output (4D RGBA)
- Channels 4-11: Static features (8D: p0-p7)

Static features:
- p0-p3 (channels 4-7): RGB+D from mip level
- p4-p7 (channels 8-11): uv_x, uv_y, sin20_y, bias

Updated:
- training/gen_identity_weights.py: Change weights[i,i+4] to weights[i,i+8]
- workspaces/main/weights/mix_p47.bin: Regenerated (not in repo)
- doc/CNN_V2.md: Add Input Channel Mapping section with full layout table

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

1 files changed, 21 insertions, 0 deletions

diff --git a/doc/CNN_V2.md b/doc/CNN_V2.md
index 577cf9e..abef606 100644
--- a/doc/CNN_V2.md
+++ b/doc/CNN_V2.md
@@ -136,6 +136,27 @@ let bias = 1.0;                     // Learned bias per output channel
 // Packed storage: [p0, p1, p2, p3, uv.x, uv.y, sin(20*uv.y), 1.0]
 ```
 
+### Input Channel Mapping
+
+**Weight tensor layout (12 input channels per layer):**
+
+| Input Channel | Feature | Description |
+|--------------|---------|-------------|
+| 0-3 | Previous layer output | 4D RGBA from prior CNN layer (or input RGBD for Layer 0) |
+| 4-11 | Static features | 8D: p0, p1, p2, p3, uv_x, uv_y, sin20_y, bias |
+
+**Static feature channel details:**
+- Channel 4 → p0 (RGB.r from mip level)
+- Channel 5 → p1 (RGB.g from mip level)
+- Channel 6 → p2 (RGB.b from mip level)
+- Channel 7 → p3 (depth or RGB channel from mip level)
+- Channel 8 → p4 (uv_x: normalized horizontal position)
+- Channel 9 → p5 (uv_y: normalized vertical position)
+- Channel 10 → p6 (sin(20*uv_y): periodic encoding)
+- Channel 11 → p7 (bias: constant 1.0)
+
+**Note:** When generating identity weights, p4-p7 correspond to input channels 8-11, not 4-7.
+
 ### Feature Rationale
 
 | Feature | Dimension | Purpose | Priority |