Add properties with docstrings to XRTDEMIterative for all public result attributes

[joyvelasquez]

Feature description

XRTDEMIterative currently exposes many of its result and grid attributes as plain instance attributes set directly with self.xxx = inside methods like solve(), create_logT_grid(), and _run_monte_carlo(). These should be converted to proper @property definitions with NumPy-style docstrings so they appear correctly in the API reference documentation and can be cross-referenced from other docs pages.

The full list of candidates:

Core grid attributes (set in create_logT_grid):

• logT — log10 temperature grid
• T — linear temperature grid in Kelvin
• dlogT — step size in log10(T)
• dlnT — step size in ln(T)
• n_bins — number of temperature bins

Initialization attributes:

• observed_channel — validated filter names
• responses — temperature response objects

Solver results (set in solve()):

• dem — DEM solution array, cm⁻⁵ K⁻¹
• chisq — chi-square value of the fit
• modeled_intensities — forward-modeled filter intensities, DN/s/pix

Monte Carlo results (set in _run_monte_carlo()):

• mc_dem — DEM solutions for all MC realizations, shape (N+1, n_T)
• mc_chisq — chi-square for each realization, shape (N+1,)
• mc_base_obs — observed intensities per realization, shape (N+1, n_channels)
• mc_mod_obs — modeled intensities per realization, shape (N+1, n_channels)

Secondary spline/internal attributes (lower priority):

• n_spl, spline_logT, spline_log_dem, abundances, interpolated_responses

Motivation

• Without @property definitions, Sphinx cannot generate API pages for these attributes and they cannot be cross-referenced from documentation using :attr: or ~xrtpy.xrt_dem_iterative.XRTDEMIterative.mc_dem style links.
• Users have no in-code documentation (docstrings) for what each result attribute contains, its shape, or its units.
• Raised during review of PR XRT DEM Iterative Solver Module #367 by @namurphy — see comment on dem_overview.rst lines 175–177.
• Consistent with the existing pattern in XRTDEMIterative where input parameters like observed_intensities, intensity_uncertainties, monte_carlo_runs, etc. are already implemented as properties with docstrings.

Implementation strategy

For each attribute:

1. Rename the internal storage to self._xxx wherever it is assigned (in solve(), create_logT_grid(), _run_monte_carlo(), etc.)
2. Add a @Property with a NumPy-style docstring specifying type, shape, and units
3. Verify that dem_plotting.py and all test files still work — since the public name stays the same, no changes should be needed there
4. Update dem_overview.rst to use Sphinx cross-reference links like ~xrtpy.xrt_dem_iterative.XRTDEMIterative.mc_dem once the properties exist

Files to modify: xrtpy/xrt_dem_iterative/dem_solver.py, docs/dem_overview.rst
Files to verify (no changes expected) - maybe : xrtpy/xrt_dem_iterative/dem_plotting.py, xrtpy/xrt_dem_iterative/test/test_dem_solver.py

Additional context

To be addressed in a follow-up PR after PR #367 (XRT DEM Iterative Solver) is merged. Extra detailed PR so it's easier to approach.