Round example model outputs to four decimal places

[Copilot]

Example model outputs now use consistent four-decimal precision across generated data, fixtures, and documentation.

• Output generation
  • Round prevalence statistics and exceedance probabilities to four decimal places before writing Parquet files.

across(c(mean, median, SD, lower_95, upper_95, starts_with("exceedance_")), ~round(.x, 4))

• Fixtures and documentation
  • Regenerate model-output fixtures with rounded values.
  • Update the README response example to match the new precision.