Infrastructure to test DaCe's codegen (in)deterministic behavior

[kotsaloscv]

Adds infrastructure to detect non-determinism in gt4py's DaCe codegen. Two new parametrized nox sessions run gt4py's own DaCe test selection twice with isolated build caches, then byte-compare the generated sources under each program's src/. The session is successful when the codegen is identical between the two runs, fails on any diff. Supports the dace_cpu, dace_gpu (CUDA), and HIP backends. Driven entirely from gt4py's own nox sessions — no external repo needed.

Layout

• noxfile.py — two new sessions, test_cartesian_determinism and test_next_determinism, parametrized over device (plus meshlib for test_next). Each calls a shared _run_dace_determinism_check helper that runs gt4py's regular DaCe pytest selection twice with GT4PY_BUILD_CACHE_DIR pointed at isolated workdirs, then invokes the comparator. Tagged dace + determinism, so nox -t determinism runs every variant.
• scripts/dace_deterministic_codegen.py — pure-stdlib comparison library + thin argparse CLI. Snapshots each cache, compares <program>/src/{cpu,cuda}/... byte-for-byte (HIP files under src/cuda/hip/ covered transparently). Distinct exception types (and CLI exit codes) for actual mismatch, unsupported backend, no programs cached, and source-not-retained — each with an actionable diagnostic. Reusable for ad-hoc comparison of any two existing caches.
• ci/cscs-ci-dace-determinism.yml — CI driver, included from cscs-ci.yml.

DaCe build folder mode

The check requires DACE_compiler_build_folder_mode=development. gt4py configures DaCe to production by default, which strips the generated src/ tree after compilation — leaving nothing to compare. The nox sessions set this env var automatically; the comparator raises NoSourceFilesObservedError with an env-var hint if it ever encounters caches built without it.

CI

New dace-determinism stage in ci/cscs-ci-dace-determinism.yml, included from cscs-ci.yml.

Local use

./noxfile.py -s 'test_next_determinism-3.10(cpu, nomesh)'   # one variant
nox -t determinism                                          # all variants
python scripts/dace_deterministic_codegen.py --run1 PATH --run2 PATH