Support block defaults for module attributes

[landeqiming666]

Summary

• support block defaults for cattr and mattr reader/writer/accessor helpers
• preserve existing class variable values instead of replacing them with block defaults
• add focused tests and QED-style examples for block defaults

Fixes #258.

Validation

• ruby -c lib/core/facets/module/mattr.rb
• ruby -c test/core/module/test_mattr.rb
• ruby -Ilib/core target smoke test for reader, accessor, memoization, and writer behavior
• git diff --check HEAD~1..HEAD