[fank]

Summary

• Match mod factorio_version to the exact game major.minor, retaining the 0.18-to-1.0 exception and missing-field default of 0.12.
• Make the updater safe to source without running updates or changing shell options.
• Add five focused Bats tests for compatibility, release selection and base patch requirements, legacy defaults, safe sourcing, and normal script execution.
• Run Bats in the existing regression CI job. No new Python tests or README changes.

Closes #613.

Validation

• All five Bats tests pass locally with Bats 1.11.0.
• Restoring the old version-selection logic in a temporary copy makes three regression tests fail as expected.
• All 21 existing regression methods pass.
• ShellCheck, actionlint, and whitespace checks pass.
• Remote CI for this commit is not yet verified.