Specify list mock semantics

[magicmark]

Summary

• add a minimal clarification that mocks beneath list-typed fields use the same mock value for each corresponding list item
• note that distinct per-item values should be modeled by mocking the list-typed field or an enclosing field
• allow arrays as mock variant data
• add a compact mock-file example for a list-typed field

Notes

This is a narrower replacement for the useful semantics explored in #2. It avoids index-style arguments and per-item targeting, since list order and nested list parents make that API fragile.

Validation

• git diff --check

Draft PR: This PR was sent by Claude and may not have yet been reviewed by markl.