Add liquid-spec for conformance testing

- Add liquid-spec gem from GitHub to :spec group
- Create spec/ruby-liquid.rb adapter for the reference implementation
- Add spec job to CI workflow to run liquid-spec tests

spec/ruby-liquid.rb

@@ -0,0 +1,34 @@
+# frozen_string_literal: true
+
+# Liquid Spec Adapter for Shopify/liquid (Ruby reference implementation)
+#
+# Run with: bundle exec liquid-spec run spec/ruby-liquid.rb
+
+$LOAD_PATH.unshift(File.expand_path('../lib', __dir__))
+require 'liquid'
+
+LiquidSpec.configure do |config|
+  # Run core Liquid specs
+  config.features = [:core]
+end
+
+# Compile a template string into a Liquid::Template
+LiquidSpec.compile do |source, options|
+  Liquid::Template.parse(source, **options)
+end
+
+# Render a compiled template with the given context
+LiquidSpec.render do |template, ctx|
+  static_registers = ctx.registers
+  registers = Liquid::Registers.new(static_registers)
+
+  context = Liquid::Context.build(
+    static_environments: ctx.environment,
+    registers: registers,
+    rethrow_errors: ctx.rethrow_errors?
+  )
+
+  context.exception_renderer = ctx.exception_renderer if ctx.exception_renderer
+
+  template.render(context)
+end