ci: gate w1a9_codegen_probe behind the std feature like its sibling examples

The example imports ndarray::simd, which is #[cfg(feature = "std")]; without
the [[example]] required-features declaration, --no-default-features CI jobs
try to build it and fail with E0432. Same trampoline every other simd-using
example already carries.

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KCGhDYoQBXs3poaR7sFuqp

Author: claude

Cargo.toml

@@ -90,6 +90,11 @@ required-features = ["std"]
 name = "codec_mode_histogram"
 required-features = ["codec"]
 
+# W1a-#9 masking-primitive codegen probe imports `ndarray::simd` (std-gated).
+[[example]]
+name = "w1a9_codegen_probe"
+required-features = ["std"]
+
 [[example]]
 name = "mc_via_shader"
 required-features = ["codec"]