fix(ci): gate AMX examples behind required-features = ["std"]

examples/{amx_gemm_bench,amx_probe,amx_rb_probe}.rs import `ndarray::simd` and
`ndarray::hpc`, both `#[cfg(feature = "std")]`. With std as a default feature
they built locally, but the `--no-default-features` CI job compiles examples
with std off → `error[E0432]: unresolved import ndarray::simd` (item configured
out, gated behind `std`). Declared the three examples with
`required-features = ["std"]` (same pattern as `ocr_benchmark`) so non-std jobs
skip them.

Verified: `cargo build -p ndarray --no-default-features
--features portable-atomic-critical-section --examples` now finishes clean
(examples skipped) instead of failing on amx_gemm_bench.

https://claude.ai/code/session_01D2WSmezQBNC3bUdHuGfGmo

Author: claude

Cargo.toml

@@ -42,6 +42,20 @@ required-features = ["std"]
 name = "splat3d_flex"
 required-features = ["splat3d"]
 
+# AMX examples import `ndarray::simd` / `ndarray::hpc`, both `#[cfg(feature =
+# "std")]`, so they must be skipped in `--no-default-features` CI jobs.
+[[example]]
+name = "amx_gemm_bench"
+required-features = ["std"]
+
+[[example]]
+name = "amx_probe"
+required-features = ["std"]
+
+[[example]]
+name = "amx_rb_probe"
+required-features = ["std"]
+
 [dependencies]
 num-integer = { workspace = true }
 num-traits = { workspace = true }