Add NXstress smoke-test demo script

[Copilot]

Summary

Implements the "by hand" smoke-test script requested in the issue, demonstrating the full feature set of the new NXstress I/O implementation.

Changes

New file: tests/scripts/cis_tests/NXstress_demo_script.py

A standalone script (ignored by pytest per the directory's README.rst) that walks through:

1. Load – opens tests/data/3393_PWHT-TD.h5 into a HidraWorkspace via HidraProjectFile.

2. Fit – calls generate_PeakCollection_from_workspace (from tests/util/peak_collection_helpers.py) with a fit_dic whose peak_label values follow the peak_tag convention required by NXstress._parse_peak_tag:

   • "Fe 311" for the first peak (2θ range 87.6 – 91.6°, d₀ = 1.08 Å)
   • "Fe 222" for the second peak (2θ range 93.5 – 95.9°, d₀ = 1.03 Å)

   These labels match the hklPhase log stored in the data file (b'Fe 311, Fe 222').

3. Write – writes the workspace and fitted list[PeakCollection] to NXstress_demo_output.nxs using NXstress as a context manager.

4. Read back – re-opens the NXstress file and performs a brief round-trip consistency check (sub-run count and peak-collection count).

Modified file: tests/util/peak_collection_helpers.py

Added the two imports that generate_PeakCollection_from_workspace already uses but were missing from the module:

from pyrs.core.workspaces import HidraWorkspace
from pyrs.peaks import FitEngineFactory as PeakFitEngineFactory  # type: ignore

Usage

python tests/scripts/cis_tests/NXstress_demo_script.py

Requires a full PyRS environment (mantid, scipy, nexusformat, etc.). Output NXstress file is written to the current working directory as NXstress_demo_output.nxs.