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

[kotsaloscv]

Adds infrastructure to detect non-determinism in gt4py's DaCe codegen.

The checker runs an icon4py test selection through nox twice with isolated
gt4py build caches, then byte-compares the generated sources under each
program's src/. Exit 0 = identical, exit 1 = different. Supports the
dace_cpu, dace_gpu (CUDA), and HIP backends.

Layout

• ci/dace_deterministic_codegen/dace_deterministic_codegen.py — the
  comparison script. Valid --selection/--component values are read
  from icon4py's noxfile.py at runtime, so it auto-tracks upstream
  changes.
• ci/dace_deterministic_codegen/bootstrap_icon4py.py — patches icon4py's
  [tool.uv.sources] to install the editable gt4py (and optionally a
  custom dace branch) into the nox session venv.
• ci/dace_deterministic_codegen/run_in_ci.sh — env-var-driven driver
  (clone + bootstrap + check) used by CI and reproducible locally.
• ci/dace_deterministic_codegen/README.md — setup, flags, exit codes,
  examples, local-repro recipe.

CI

New dace-determinism stage in ci/cscs-ci-dace-determinism.yml, included
from cscs-ci.yml. Two jobs on the santis gh200 runner (CUDA + CPU), each
a 4-cell matrix over component {dycore, advection, diffusion, muphys}
with selection=stencils and grid=icon_regional. allow_failure: true
while the toolchain stabilizes — to be dropped once main stays green.

Setting DACE_REPO/DACE_REF points a run at an unmerged dace branch
(e.g. the deterministic-codegen work); when both are empty, dace resolves
through icon4py's existing pin.

Per-cell artifacts (run1/run2 caches, diffs/, report.txt) are
retained for 1 month.