ci: gate signature_pde_bench behind the std feature

hpc (and therefore signature_pde_sweep) is #[cfg(feature = "std")]-gated,
but the new example used it unconditionally. CI runs
`cargo test --package ndarray --no-default-features`, which builds every
example by default regardless of default-features — breaking with
"cannot find `hpc` in `ndarray`". Fix mirrors every other hpc-dependent
example in this Cargo.toml (entropy_ladder_probe, instrument_mtmm_probe,
cakes_grail_probe, ...): required-features = ["std"], so cargo skips the
target instead of failing to compile it. Reproduced the failure locally
with the exact CI invocation, confirmed the fix skips (not fails) the
build under --no-default-features, and confirmed the default (std) build
still compiles and runs the example, and the lib tests still pass 5/5.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KCGhDYoQBXs3poaR7sFuqp

Author: claude

Cargo.toml

@@ -135,6 +135,10 @@ required-features = ["std"]
 name = "instrument_mtmm_probe"
 required-features = ["std"]
 
+[[example]]
+name = "signature_pde_bench"
+required-features = ["std"]
+
 [[example]]
 name = "cakes_grail_probe"
 required-features = ["std"]