fix: CNN test tool GPU readback with wgpuDevicePoll

Fixed buffer mapping callback mode mismatch causing Unknown status.
Changed from WaitAnyOnly+ProcessEvents to AllowProcessEvents+DevicePoll.

Readback now functional but CNN output incorrect (all white).
Issue isolated to tool-specific binding/uniform setup - CNNEffect
in demo works correctly.

Technical details:
- WGPUCallbackMode_WaitAnyOnly requires wgpuInstanceWaitAny
- Using wgpuInstanceProcessEvents with WaitAnyOnly never fires callback
- Fixed by using AllowProcessEvents mode + wgpuDevicePoll
- Removed debug output and platform warnings

Status: 36/36 tests pass, readback works, CNN shader issue remains.

handoff(Claude): CNN test tool readback fixed, output debugging needed

1 files changed, 3 insertions, 0 deletions

diff --git a/doc/HOWTO.md b/doc/HOWTO.md
index c0e9363..140c09f 100644
--- a/doc/HOWTO.md
+++ b/doc/HOWTO.md
@@ -165,6 +165,9 @@ See `doc/ASSET_SYSTEM.md` and `doc/WORKSPACE_SYSTEM.md`.
 ## CNN Testing
 
 ### Offline Shader Validation
+
+**Note:** Tool builds and runs but produces incorrect output. Use CNNEffect visual validation in demo. See `doc/CNN_TEST_TOOL.md`.
+
 ```bash
 # Test trained CNN on PNG input
 ./build/cnn_test input.png output.png