Feature summary (what you would like to be able to do and where):
Add a line like "[...] You can browse it at $1, and if you wish to submit patches, see [[mw:Extension:WikiLambda]] for instructions." to the interface message wikilambda-implementation-selector-none (used for built-ins e.g. Z950), and when expanding it, pass in a link to the relevant source file.

Use case(s) (list the steps that you performed to discover that problem, and describe the actual underlying problem which you want to solve. Do not describe only a solution):
Without the source for these Implementations, that leaves only the Test cases (but for T413841) and the description field to document their behaviour (considering built-ins are the preferred, and often only, Implementations of their respective Functions).

Benefits (why should this be implemented?):
Having easy access to the source may allow editors to identify un- or under-documented behaviours and write Test cases targeting them.
It may encourage contributions to WikiLambda.