#835: Accept property arrays in Conclude#follow

[gemshrine]

Fbe::Conclude#follow documents Array<String> input but calls strip on the argument, so a property array raises NoMethodError before the conclusion runs.

The method now accepts both arrays of property names and the existing whitespace-separated string form. It rejects unsupported input types and non-string array entries with Fbe::Error, while preserving the existing prohibition on following what and details.

Regression coverage exercises array input through a real conclusion and verifies that invalid array entries raise the documented error. Existing tests continue to cover the string form. The repository suite runs these cases with bundle exec rake test.

Fixes #835