Update and unify examples

[LeonidElkin]

Summary

Update and unify project examples so that they follow the same style, use current public APIs, and are easy to run independently.

Examples are often the first entry point for new users, so they should be consistent and up to date.

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Problems

Current examples may differ in import style, family registration style, naming conventions, reproducibility, and whether they can be run as standalone scripts.

Some examples may also use outdated or non-recommended API patterns.

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Expected style

Examples should be:

1. Minimal but complete.
2. Deterministic when randomness is involved.
3. Consistent in naming and formatting.
4. Runnable directly as standalone scripts.
5. Focused on one concept per example.
6. Compatible with the current public API.

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Scope

Review examples related to:

1. Built-in families.
2. Parametrizations.
3. Analytical characteristics.
4. Sampling.
5. UNU.RAN sampling.
6. Transformations.
7. Constraints/support usage.
8. Fitting, if examples exist.

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Acceptance criteria

1. Existing examples are reviewed.
2. Outdated imports and API calls are updated.
3. Sampling examples use explicit seeds where possible.
4. Examples follow a consistent structure.
5. Examples are grouped or named consistently.
6. Each example can be run independently.
7. Documentation references are updated if paths or names change.