fix(render): preserve scope chain when self: bound on render tag

The render tag wrote `self:` attributes directly into @scopes[0] under
key 'self', which shadowed the SelfDrop. `self[var]` lookups inside the
snippet then did strict key access on the bound object instead of
walking the scope chain, resolving to nil for any key not in the object.

SelfDrop now holds an optional `bound_self`; `[]` and `key?` consult it
first via duck typing (matches lib/liquid/variable_lookup.rb), then fall
through to the scope-chain walk on miss. Render#render_tag routes
attribute key Expression::SELF to inner_context.self_drop.bound_self=
instead of the generic scope write. Lookup order is bound-first;
existing self: users' hits stay intact, previously-nil misses now
resolve via fallthrough.

PR #2060 introduced the SelfDrop and bare-bracket prohibition but had
no coverage for the {% render 'snippet', self: obj %} interaction.
Adds 8 tests including two-deep nested renders with leak detection
and a composite chain combining renders, top-level self[var], and
regular variables.

Discovered while investigating SFR strict-parser migration parity
diffs.

lib/liquid/context.rb

@@ -200,6 +200,10 @@ module Liquid
       object.respond_to?(:evaluate) ? object.evaluate(self) : object
     end
 
+    def self_drop
+      @self_drop ||= SelfDrop.new(self)
+    end
+
     # Fetches an object starting at the local scope and then moving up the hierachy
     def find_variable(key, raise_on_not_found: true)
       # This was changed from find() to find_index() because this is a very hot
@@ -208,7 +212,7 @@ module Liquid
 
       # `self` resolves to a SelfDrop (enabling `self['var']` lookups),
       # but only when it hasn't been explicitly assigned as a local variable.
-      return SelfDrop.new(self) if key == Expression::SELF && !index
+      return self_drop if key == Expression::SELF && !index
 
       variable = if index
         lookup_and_evaluate(@scopes[index], key, raise_on_not_found: raise_on_not_found)

lib/liquid/self_drop.rb

@@ -16,23 +16,42 @@ module Liquid
   #   then the local value takes precedence over the `self` object.
   # @liquid_access global
   class SelfDrop < Drop
+    attr_accessor :bound_self
+
     def initialize(context)
       super()
       @context = context
+      @bound_self = nil
     end
 
     def [](key)
-      @context.find_variable(key)
+      if @bound_self && bound_has?(key)
+        bound_lookup(key)
+      else
+        @context.find_variable(key)
+      end
     rescue UndefinedVariable
       nil
     end
 
     def key?(key)
-      @context.variable_defined?(key)
+      (@bound_self && bound_has?(key)) || @context.variable_defined?(key)
     end
 
     def to_liquid
       self
     end
+
+    private
+
+    def bound_has?(key)
+      @bound_self.respond_to?(:key?) && @bound_self.key?(key)
+    end
+
+    def bound_lookup(key)
+      return unless @bound_self.respond_to?(:[])
+
+      @bound_self[key]
+    end
   end
 end

lib/liquid/tags/render.rb

@@ -66,7 +66,12 @@ module Liquid
         inner_context['forloop']    = forloop if forloop
 
         @attributes.each do |key, value|
-          inner_context[key] = context.evaluate(value)
+          evaluated = context.evaluate(value)
+          if key == Expression::SELF
+            inner_context.self_drop.bound_self = evaluated
+          else
+            inner_context[key] = evaluated
+          end
         end
         inner_context[context_variable_name] = var unless var.nil?
         partial.render_to_output_buffer(inner_context, output)