refactor(cnn): isolate CNN v2 to cnn_v2/ subdirectory

Move all CNN v2 files to dedicated cnn_v2/ directory to prepare for CNN v3 development. Zero functional changes.

Structure:
- cnn_v2/src/ - C++ effect implementation
- cnn_v2/shaders/ - WGSL shaders (6 files)
- cnn_v2/weights/ - Binary weights (3 files)
- cnn_v2/training/ - Python training scripts (4 files)
- cnn_v2/scripts/ - Shell scripts (train_cnn_v2_full.sh)
- cnn_v2/tools/ - Validation tools (HTML)
- cnn_v2/docs/ - Documentation (4 markdown files)

Changes:
- Update CMake source list to cnn_v2/src/cnn_v2_effect.cc
- Update assets.txt with relative paths to cnn_v2/
- Update includes to ../../cnn_v2/src/cnn_v2_effect.h
- Add PROJECT_ROOT resolution to Python/shell scripts
- Update doc references in HOWTO.md, TODO.md
- Add cnn_v2/README.md

Verification: 34/34 tests passing, demo runs correctly.

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

1 files changed, 75 insertions, 0 deletions

diff --git a/cnn_v2/shaders/cnn_v2_static.wgsl b/cnn_v2/shaders/cnn_v2_static.wgsl
new file mode 100644
index 0000000..309e832
--- /dev/null
+++ b/cnn_v2/shaders/cnn_v2_static.wgsl
@@ -0,0 +1,75 @@
+// CNN v2 Static Features Compute Shader
+// Generates 8D parametric features: [p0, p1, p2, p3, uv.x, uv.y, sin20_y, bias]
+// p0-p3: Parametric features from specified mip level (0=mip0, 1=mip1, 2=mip2, 3=mip3)
+// Note: Input image RGBD (mip0) fed separately to Layer 0
+//
+// TODO: Binary format should support arbitrary layout and ordering for feature vector (7D).
+//       Current layout is hardcoded. Future versions should allow runtime-specified
+//       feature combinations (e.g., [R, G, B, dx, dy, uv_x, bias] or custom encodings).
+
+struct StaticFeatureParams {
+  mip_level: u32,
+  padding0: u32,
+  padding1: u32,
+  padding2: u32,
+}
+
+@group(0) @binding(0) var input_tex: texture_2d<f32>;
+@group(0) @binding(1) var input_tex_mip1: texture_2d<f32>;
+@group(0) @binding(2) var input_tex_mip2: texture_2d<f32>;
+@group(0) @binding(3) var depth_tex: texture_2d<f32>;
+@group(0) @binding(4) var output_tex: texture_storage_2d<rgba32uint, write>;
+@group(0) @binding(5) var<uniform> params: StaticFeatureParams;
+@group(0) @binding(6) var linear_sampler: sampler;
+
+@compute @workgroup_size(8, 8)
+fn main(@builtin(global_invocation_id) id: vec3<u32>) {
+  let coord = vec2<i32>(id.xy);
+  let dims = textureDimensions(input_tex);
+
+  if (coord.x >= i32(dims.x) || coord.y >= i32(dims.y)) {
+    return;
+  }
+
+  // Parametric features (p0-p3) - bilinear sample from specified mip level
+  // Use UV coordinates for bilinear interpolation
+  // Note: Use textureSampleLevel (not textureSample) in compute shaders
+  let uv = (vec2<f32>(coord) + 0.5) / vec2<f32>(dims);
+  var rgba: vec4<f32>;
+  if (params.mip_level == 0u) {
+    rgba = textureSampleLevel(input_tex, linear_sampler, uv, 0.0);
+  } else if (params.mip_level == 1u) {
+    rgba = textureSampleLevel(input_tex_mip1, linear_sampler, uv, 0.0);
+  } else if (params.mip_level == 2u) {
+    rgba = textureSampleLevel(input_tex_mip2, linear_sampler, uv, 0.0);
+  } else {
+    // Mip 3 or higher: use mip 2 as fallback
+    rgba = textureSampleLevel(input_tex_mip2, linear_sampler, uv, 0.0);
+  }
+
+  let p0 = rgba.r;
+  let p1 = rgba.g;
+  let p2 = rgba.b;
+  let p3 = textureLoad(depth_tex, coord, 0).r;
+
+  // UV coordinates (normalized [0,1], top-left origin - matches training)
+  let uv_x = f32(coord.x) / f32(dims.x);
+  let uv_y = f32(coord.y) / f32(dims.y);
+
+  // Multi-frequency position encoding
+  let sin20_y = sin(20.0 * uv_y);
+
+  // Bias dimension (always 1.0)
+  let bias = 1.0;
+
+  // Pack 8×f16 into 4×u32 (rgba32uint)
+  // [p0, p1, p2, p3, uv_x, uv_y, sin20_y, bias]
+  let packed = vec4<u32>(
+    pack2x16float(vec2<f32>(p0, p1)),
+    pack2x16float(vec2<f32>(p2, p3)),
+    pack2x16float(vec2<f32>(uv_x, uv_y)),
+    pack2x16float(vec2<f32>(sin20_y, bias))
+  );
+
+  textureStore(output_tex, coord, packed);
+}