Function validate_output 

pub fn validate_output(result: &SolverResult) -> Result<(), ValidationError>

Validate a solver result after computation completes.

This catches silent numerical corruption that may have occurred during iteration:

1. No NaN or Inf in the solution vector.
2. The residual norm is finite.
3. At least one iteration was performed.

Errors

Returns ValidationError if the output is corrupted.