[fix] Format singleton class expression instead of hardcoding self (#876)

`format_singleton_class_node` emitted a hardcoded `self` identifier
regardless of the actual receiver expression, silently rewriting
`class << obj` to `class << self`. Now formats the AST expression node
directly, preserving whatever receiver the user wrote.

Fixes #874

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: thomasklemm

fixtures/small/singleton_class_expression_actual.rb

@@ -0,0 +1,31 @@
+class << self
+  def foo; end
+end
+
+class << receiver
+  def foo; :bar; end
+end
+
+class << @ivar
+  def foo; end
+end
+
+class << @@cvar
+  def foo; end
+end
+
+class << $global
+  def foo; end
+end
+
+class << Foo
+  def foo; end
+end
+
+class << SomeModule::CONST
+  def foo; end
+end
+
+class << foo.bar
+  def foo; end
+end

fixtures/small/singleton_class_expression_expected.rb

@@ -0,0 +1,40 @@
+class << self
+  def foo
+  end
+end
+
+class << receiver
+  def foo
+    :bar
+  end
+end
+
+class << @ivar
+  def foo
+  end
+end
+
+class << @@cvar
+  def foo
+  end
+end
+
+class << $global
+  def foo
+  end
+end
+
+class << Foo
+  def foo
+  end
+end
+
+class << SomeModule::CONST
+  def foo
+  end
+end
+
+class << foo.bar
+  def foo
+  end
+end

librubyfmt/src/format_prism.rs

@@ -4765,7 +4765,9 @@ fn format_singleton_class_node<'src>(
     ps.emit_space();
     ps.emit_ident(b"<<");
     ps.emit_space();
-    ps.emit_ident(b"self");
+    ps.with_start_of_line(false, |ps| {
+        format_node(ps, singleton_class_node.expression())
+    });
 
     ps.new_block(|ps| {
         ps.with_start_of_line(true, |ps| {