Iterate#sort_by crashes when matching facts have different value types

[gemshrine]

Fbe::Iterate#sort_by crashes when matching facts use different valid value types for the selected property. Factbase permits a property to contain values such as both integers and strings, but the iterator collects those values and calls Ruby's Array#sort! without a mixed-type comparison strategy.

Steps to reproduce:

1. Add matching facts whose rank property contains values of different types, such as integer 2 and string "alpha".
2. Configure Fbe::Iterate with sort_by('rank') and a query matching both facts.
3. Run the iteration.

Actual result: the first call to values[repo].next builds the list with .filter_map { _1[@sorting]&.first }.uniq.sort!.each. Ruby cannot compare an Integer with a String, so sorting raises ArgumentError and the repository scan aborts before the block processes its results.

Expected result: a mixed-type property should not crash the iterator. It should either use a documented deterministic ordering for valid Factbase values or reject the heterogeneous values with an actionable validation error before iteration begins.

Technical evidence: lib/fbe/iterate.rb implements sort_by by sorting raw property values in Ruby. Factbase allows strings, integers, floats, times, and booleans as valid property values, and a property can hold multiple values. The sort_by API currently has no requirement that all selected values share one type.