fix(qdp-core): link cudart locally and stub FFI when toolkit is absent

[andrewmusselman]

Closes #1318 .

qdp-core/build.rs (extended; it already existed for protoc) now:

• probes for nvcc with the same logic as qdp-kernels/build.rs,
• emits cargo:rustc-link-search=native=$CUDA_PATH/lib64 and
  cargo:rustc-link-lib=cudart when found,
• emits cargo:rustc-cfg=qdp_no_cuda when not found, with a clear
  cargo:warning pointing at the toolkit install,
• respects QDP_NO_CUDA=1 for explicit forcing (matching qdp-kernels).
  qdp-core/src/gpu/cuda_ffi.rs wraps the extern "C" block in
  #[cfg(not(qdp_no_cuda))] and adds matching #[cfg(qdp_no_cuda)] mod no_cuda_stubs { ... } with pub(crate) unsafe fn stubs for all 14
  declared functions. Each stub returns 999 — the same sentinel
  qdp-kernels uses for its kernel-launcher stubs — so existing caller
  error paths (if ret != 0 { return Err(...) }) surface a clean runtime
  error if anyone calls a CUDA function on a no-toolkit build, instead of
  failing at link time. The whole stub module is wrapped in a single
  #[allow(non_snake_case)] since the originals are camelCase to match
  the real CUDA Runtime API.

Cross-crate behaviour after this PR:

environment	qdp-kernels	qdp-core	result
toolkit installed	links cudart	links cudart	normal GPU build
driver only / macOS / CI	stub launchers	stub Runtime API	links; runtime err 999
QDP_NO_CUDA=1	stub launchers	stub Runtime API	links; runtime err 999

Verified on Linux + CUDA 12.4: cargo build --workspace --tests --exclude qdp-python succeeds both with and without QDP_NO_CUDA=1;
make test_rust runs full integration tests on the GPU; 12 lint
warnings introduced by the stubs are silenced.

[ryankert01]

Like the high-level idea:

• With CUDA toolkit installed: normal GPU-enabled build.
• Without CUDA toolkit: build still links successfully, but CUDA calls return a controlled runtime error.
• With QDP_NO_CUDA=1: same forced no-CUDA behavior.

[ryankert01]

LGTM. Two small things worth doing. Also, needs to fix the pre-commit.

[ryankert01]

output().is_ok() is true even if nvcc exits non-zero — a half-installed nvcc would set has_cuda = true and fall through to a link error. Suggest .map(|o| o.status.success()).unwrap_or(false). Same idiom in qdp-kernels/build.rs:177; fix both so they can't disagree.

[ryankert01]

Missing cargo:rerun-if-env-changed=PATH. The whole decision hinges on nvcc-on-PATH, so installing CUDA after a stub build won't re-trigger the script until cargo clean. Same gap in qdp-kernels.