Add Liquid::Box for secure sandboxed template execution

Introduces Liquid::Box which wraps Ruby 4.0's Ruby::Box for secure
template execution. On Ruby < 4.0, provides a polyfill with security
warnings.

Key features:
- Detects Ruby::Box availability at load time
- Loads safe libraries (CGI, Base64, BigDecimal) into sandbox
- Neuters dangerous methods (file IO, process control, eval, etc.)
- Preserves user constants defined before lock!
- Provides setup_gem_load_paths! to enable gem requires in box

Security model: It is safe to expose side-effect-free, non-IO methods
that don't leak objects with dangerous methods. The sandbox blocks
capabilities, not data.

lib/liquid.rb

@@ -89,3 +89,4 @@ require 'liquid/partial_cache'
 require 'liquid/usage'
 require 'liquid/registers'
 require 'liquid/template_factory'
+require 'liquid/box'