fix(ruby): emit mixes_in for include/prepend inside class << self

[Synvoya]

Bug

An include/prepend inside a class << self block injects the module's instance methods as the enclosing class's own singleton (class) methods — the exact mixin relationship that extend <Mod> expresses in the class body, which the extractor already captures as a mixes_in edge (#1668).

The Ruby mixin scan in _extract_generic only walked the class/module body's direct children, so an include/prepend nested one level down inside a class << self singleton-class block was silently skipped and produced no edge. This drops the heritage link for the canonical "add class methods via a module" idiom.

Minimal repro

# concern.rb
module Announceable
  def announce; end
end

# broadcaster.rb
class Broadcaster
  class << self
    include Announceable   # -> Broadcaster gains `announce` as a CLASS method
  end
end

• Before: no mixes_in edge — _mixes_in(graph) == set()
• After: ("Broadcaster", "Announceable") in mixes_in, identical to the semantically-equivalent extend Announceable

singleton_class was previously unreferenced anywhere in the codebase, so this form had no handling at all.

Fix

Scan class << self blocks in addition to the class/module body, reusing the existing constant-argument filter and the single-owner resolver guard — no new resolution path. The inner statement loop is unchanged; only the set of statements it iterates is widened.

Guard: only self counts. class << other opens a different object's singleton class, not the enclosing class's, so an include there stays unlinked (covered by a negative test).

Tests

Added to tests/test_ruby_resolution.py (alongside the #1668 mixin specs):

• test_include_in_singleton_class_emits_mixes_in — the fix; fails before, passes after.
• test_include_in_other_object_singleton_class_emits_no_mixin — locks the class << other guard (no over-emission).

tests/test_ruby_resolution.py + tests/test_languages.py: 338 passed, 13 skipped (skips are the optional tree-sitter-dm grammar). No cross-language impact — the changed block is inside the config.ts_module == "tree_sitter_ruby" guard.