refactor: cache fit_all results on instance and remove fit_results parameter from best_fit

[MAfarrag]

Problem

best_fit has a fit_results parameter so users can pass pre-computed fit_all results to avoid re-fitting. This is an internal optimization concern leaking into the public API. It forces the user to manually wire data between the two methods:

results = dist.fit_all()
best_name, best_info = dist.best_fit(fit_results=results)  # manual plumbing

The parameter also creates two calling modes (with/without fit_results), making the method signature harder to understand: method and distributions are silently ignored when fit_results is provided, which is not obvious to the caller.

Proposed Solution

1. fit_all stores its results on self._fit_results after computation.
2. best_fit reads from self._fit_results automatically. If no cached results exist, it calls fit_all internally.
3. Remove the fit_results parameter from best_fit entirely.

Before

dist = Distributions(data=data)
results = dist.fit_all()
best_name, best_info = dist.best_fit(fit_results=results)

After

dist = Distributions(data=data)
dist.fit_all()                              # cached on self
best_name, best_info = dist.best_fit()      # uses cache automatically

# or skip fit_all entirely:
best_name, best_info = dist.best_fit()      # calls fit_all internally, caches

Considerations

• Calling fit_all again with different parameters (e.g., different method or distributions) should overwrite the cache. This is the expected behavior.
• best_fit should still accept method, distributions, and criterion parameters. If best_fit is called and no cache exists, it calls fit_all(method=method, distributions=distributions) to populate the cache.
• Initialize self._fit_results = None in __init__.

Scope

• Add self._fit_results = None to __init__
• fit_all: store results in self._fit_results before returning
• best_fit: remove fit_results parameter, read from self._fit_results
• Update tests (remove all fit_results= usage, test caching behavior)
• Update docstring examples

Depends On

• refactor: replace untyped nested dicts in fit_all return with FitResult dataclass #152 (FitResult dataclass — do both in the same PR or sequence them)