Builds a compact before/after snippet for a mutation.

Reporters use this to show the actual code change introduced by each mutation, making survived or suspicious mutants easy to reproduce. Snippets are rendered from the mutation's :original_ast and :ast via Macro.to_string/1, falling back to inspect/1 for abstract code that the Elixir formatter cannot stringify (e.g. Erlang forms).

of/1 returns nil when the mutation does not carry both the original and mutated AST, so callers can omit the patch gracefully.