[apiology]

Claude: This PR was written by Claude Code on behalf of @apiology.

Problem:

With SOLARGRAPH_ASSERTS=on, an ivar assigned in a module_function method crashes mapping:

module Functions
  module_function

  def foo
    # source not provided -   Solargraph::Pin::Namespace (RuntimeError)
    @foo ||= {}
  end
end

This aborts strong typechecking of any project that memoizes in a module_function method, including CacheDir.plugins in castwide#1407.

Solution:

Give the module-scope closure pin source: :parser, as its sibling pins in ivasgn_node.rb already have.