NoDuplicateLegacyClassesTest

Proves every legacy class moved into :compat exists exactly once on the :compat runtime classpath and that the io.github.wax911.library.* type aliases produce no runtime classes of their own (Kotlin type aliases erase at compile time).

Duplicates would appear when a legacy implementation was left behind in another module (:api, :runtime, :serialization-gson, :serialization-kotlinx) while a copy moved to :compat.