Resolve instance variable type when assigned only in an ancestor class's method

[apiology]

Claude: written on behalf of @apiology.

Fixes #1261

Problem: An instance variable assigned only in an ancestor class's method gets no inferred type in the descendant, so every read of it fails at --level strong.

class WidgetBase
  def run
    @name = 'set-in-run'
  end
end

class Widget < WidgetBase
  # @return [void]
  def test_no_block
    run
    puts @name.upcase   # Unresolved call to @name
  end
end

This hits any class hierarchy where a base class sets up state its subclasses read, including when the two live in different files.

Solution: Pin::InstanceVariable overrides visible_in_closure? to accept a candidate when the lexical walk bottoms out at a non-matching Namespace — ApiMap#get_instance_variable_pins has already restricted candidates to the reading namespace and its ancestors — and overrides visible_at? so the same-file requirement applies only when presence (flow-sensitive narrowing) is set.

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