Preserve sampling chains in saved results

[Fstarnb]

Description

Saved SamplingResult data now includes the retained sampling chains. Result.save_data writes them for JSON, MATLAB, and pickle formats. Result.load_data restores them as a SamplingResult, with chains and all_samples populated. Before this change the chains were not saved, so a loaded sampling result had no chain data.

This supersedes #987, which was closed earlier.

Fixes #898

Design notes

• The issue suggested rebuilding the chains from x_model. This PR stores the chains and their shape explicitly. Evaluation logs do not record accepted chain states or discarded warm-up iterations, so rebuilding them would not be exact.
• Result.load_data keeps its public behaviour and return type. The loading code is split into the internal helpers _load_data_dict and _from_data_dict.
• Loading a file that has no chains (for example, a file written before this change) raises a ValueError rather than guessing the chains.
• The problem metadata of a loaded result holds parameter names only. This is noted in the docstring, because rebuilding a full problem needs the original definition.

Type of change

• Bug fix

Testing

• tests/unit/test_sampling.py and tests/unit/test_save_data.py: 93 passed.
• tests/unit/test_optimisation.py -k "save or load or Result": 5 passed.
• Full unit suite (tests/unit): 311 passed, 6 skipped, 0 failed.
• ruff check and ruff format --check on the changed files: clean.
• Not run: nox -s pre-commit (ran ruff directly) and nox -s doctest.

Changelog entry added under Features.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.94%. Comparing base (3171a07) to head (086844c).
⚠️ Report is 19 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #988      +/-   ##
===========================================
+ Coverage    92.13%   92.94%   +0.80%     
===========================================
  Files           74       84      +10     
  Lines         5799     6818    +1019     
===========================================
+ Hits          5343     6337     +994     
- Misses         456      481      +25     

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