fix(cnn_v3): native OPEN_EXR_MULTILAYER + quiet render + flexible channel names

blender_export.py:
- Replace broken compositor FileOutput approach with native OPEN_EXR_MULTILAYER
  render output; all enabled passes included automatically, no socket wiring needed
- Suppress Fra:/Mem: render spam via os.dup2 fd redirect; per-frame progress
  printed to stderr via render_post handler

pack_blender_sample.py:
- get_pass_r: try .R/.X/.Y/.Z/'' suffixes + aliases param for Depth→Z fallback
- combined_rgba loaded once via ("Combined","Image") loop; shared by transp+target
- Remove unused sys import

HOW_TO_CNN.md: update channel table to native EXR naming (Depth.Z, IndexOB.X,
Shadow.X), fix example command, note Shadow defaults to 255 when absent

handoff(Gemini): blender pipeline now produces correct multilayer EXR with all
G-buffer passes; pack script handles native channel naming

1 files changed, 26 insertions, 22 deletions

diff --git a/cnn_v3/docs/HOW_TO_CNN.md b/cnn_v3/docs/HOW_TO_CNN.md
index 3013e1c..69bbf6f 100644
--- a/cnn_v3/docs/HOW_TO_CNN.md
+++ b/cnn_v3/docs/HOW_TO_CNN.md
@@ -138,49 +138,53 @@ Produces all 20 feature channels including normals, depth, mat IDs, and shadow.
 
 #### Blender requirements
 
-- Blender 3.x+ or 5.x+, Cycles render engine (5.x API differences handled automatically)
-- Object indices set: *Properties → Object → Relations → Object Index* must be > 0
+- **Blender 4.5 LTS** recommended (`blender4` alias); 5.x also works
+- Cycles render engine (set automatically by the script)
+- Object indices set: *Properties → Object → Relations → Object Index* > 0
   for objects you want tracked in `matid` (IndexOB pass)
 
 #### Step 1 — Render EXRs
 
 ```bash
-blender -b scene.blend -P cnn_v3/training/blender_export.py -- \
-    --output /tmp/renders/ \
+cd cnn_v3/training
+blender4 -b input_3d/scene.blend -P blender_export.py -- \
+    --output tmp/renders/frames \
     --width 640 --height 360 \
-    --start-frame 1 --end-frame 200
+    --start-frame 1 --end-frame 200 \
+    --view-layer RenderLayer
 ```
 
-The `--` separator is **required**; arguments after it are passed to the Python script,
-not to Blender. `--output` is the base directory for the compositor File Output node;
-Blender appends the slot name and frame number automatically (e.g. `Combined0001.exr`).
-`//` is a Blender shorthand for the directory containing the `.blend` file.
+The `--` separator is **required**. `blender_export.py` uses native
+`OPEN_EXR_MULTILAYER` render output — all enabled passes are written
+automatically. One file per frame: `{output}/0001.exr`, `0002.exr`, …
+Render progress (`Fra:/Mem:` spam) is suppressed; per-frame status goes to stderr.
 
 **Available flags:**
 
 | Flag | Default | Notes |
 |------|---------|-------|
-| `--output PATH` | `//renders/` | Base output directory; `//` = blend file directory |
+| `--output PATH` | `//renders/` | Output directory; `//` = blend file directory |
 | `--width N` | 640 | Render resolution |
 | `--height N` | 360 | Render resolution |
 | `--start-frame N` | scene start | First frame |
 | `--end-frame N` | scene end | Last frame |
 | `--view-layer NAME` | first layer | View layer name; pass `?` to list available layers |
 
-**Render pass → CNN channel mapping:**
+**Render pass → EXR channel → CNN file:**
 
-| Blender pass | EXR channels | CNN use |
-|-------------|-------------|---------|
-| Combined | `.R .G .B .A` | `target.png` (beauty, sRGB-converted) |
-| DiffCol | `.R .G .B` | `albedo.png` (linear → sRGB gamma 2.2) |
-| Normal | `.X .Y .Z` | `normal.png` (world-space, oct-encoded to RG) |
-| Z | `.R` | `depth.png` (mapped as 1/(z+1) → uint16) |
-| IndexOB | `.R` | `matid.png` (object index, clamped uint8) |
-| Shadow | `.R` | `shadow.png` (255 = lit, 0 = shadowed) |
-| Combined alpha | `.A` | `transp.png` (inverted: 0 = opaque) |
+| Blender pass | Native EXR channel | CNN file |
+|-------------|--------------------|---------|
+| Combined | `Combined.R/G/B/A` | `target.png` (beauty, linear→sRGB) |
+| DiffCol | `DiffCol.R/G/B` | `albedo.png` (linear→sRGB γ2.2) |
+| Normal | `Normal.X/Y/Z` | `normal.png` (oct-encoded RG) |
+| Depth | `Depth.Z` | `depth.png` (1/(z+1) → uint16) |
+| IndexOB | `IndexOB.X` | `matid.png` (object index, uint8) |
+| Shadow | `Shadow.X` | `shadow.png` (255=lit; defaults to 255 if absent) |
+| Combined alpha | `Combined.A` | `transp.png` (1−alpha, 0=opaque) |
 
-**Pitfall:** Blender `Normal` pass uses `.X .Y .Z` channel names in the EXR, not `.R .G .B`.
-`pack_blender_sample.py` handles both naming conventions automatically.
+**Note on Shadow pass:** Blender's Cycles Shadow pass may be absent for scenes
+without shadow-casting lights or catcher objects; `pack_blender_sample.py` defaults
+to 1.0 (fully lit) when the channel is missing.
 
 #### Step 2 — Pack EXRs into sample directories