Add LR runtime module with pre-loaded helpers

The LR module provides all helper methods for compiled templates.
It is loaded into the sandbox BEFORE lock!, so helpers are defined
once and shared by all templates.

Helpers include:
- Type conversion: to_s, to_number, to_integer
- Output: output (handles nil, arrays, BigDecimal formatting)
- Lookup: lookup (hash/array access, Drop context support)
- Encoding: escape_html, url_encode/decode, base64_encode/decode
- Filters: truncate, truncatewords, slice, date, default, etc.

Method references to CGI, Base64, BigDecimal are captured at load
time, enabling safe use of these libraries within the sandbox.

Design principle: Maximize work in pre-loaded runtime, minimize
generated template code.

lib/liquid/compile/runtime.rb

@@ -0,0 +1,237 @@
+# frozen_string_literal: true
+
+# Liquid Compiled Template Runtime
+#
+# This module provides ALL helpers for compiled Liquid templates.
+# It is loaded into the sandbox BEFORE lock!, so these methods
+# are available to all compiled templates.
+#
+# Templates reference these as LR.method_name(args)
+#
+# == Security Note
+#
+# It is SAFE to expose side-effect-free, non-IO methods that don't leak
+# objects with dangerous methods. For example:
+#
+#   - CGI.escapeHTML(str) -> returns a String (safe)
+#   - Base64.strict_encode64(str) -> returns a String (safe)
+#   - BigDecimal(str) -> returns a Numeric (safe)
+#
+# These are pure functions that take values and return values.
+# They don't provide any capability to escape the sandbox.
+#
+# We capture METHOD REFERENCES (e.g., CGI.method(:escapeHTML)) before the
+# sandbox locks. This means templates can use the functionality without
+# having direct access to the CGI, Base64, or BigDecimal constants.
+#
+# Dependencies (CGI, Base64, BigDecimal) are loaded by box.rb into the
+# sandbox BEFORE this file runs. The constants are preserved after lock.
+
+module LR
+  # === Type Conversion ===
+
+  # Convert to string like Liquid does
+  def self.to_s(obj)
+    case obj
+    when nil then ''
+    when Array then obj.join
+    else obj.to_s
+    end
+  end
+
+  # Convert to number for arithmetic
+  # Use Kernel.BigDecimal to access from root namespace
+  def self.to_number(obj)
+    case obj
+    when Numeric then obj
+    when String
+      if obj.strip =~ /\A-?\d+\.\d+\z/
+        Kernel.BigDecimal(obj)
+      else
+        obj.to_i
+      end
+    else 0
+    end
+  end
+
+  # Convert to integer
+  def self.to_integer(obj)
+    return obj if obj.is_a?(Integer)
+    Integer(obj.to_s) rescue 0
+  end
+
+  # === Truthiness ===
+
+  # Liquid truthiness: only nil and false are falsy
+  def self.truthy?(obj)
+    obj != nil && obj != false
+  end
+
+  # === Output ===
+
+  # Output a value, converting to string safely (handles arrays recursively)
+  def self.output(obj)
+    case obj
+    when nil then ''
+    when Array then obj.map { |o| output(o) }.join
+    when BigDecimal
+      # Format BigDecimal like Liquid does - avoid scientific notation
+      obj.to_s('F')
+    else obj.to_s
+    end
+  end
+
+  # === Lookup ===
+
+  # Variable lookup - handles hash/array access, method calls, to_liquid, and drop context
+  def self.lookup(obj, key, context = nil)
+    return nil if obj.nil?
+
+    # Set context on Drops BEFORE accessing their methods
+    obj = obj.to_liquid if obj.respond_to?(:to_liquid)
+    obj.context = context if context && obj.respond_to?(:context=)
+
+    # Perform the lookup
+    result = if obj.respond_to?(:[]) && (obj.respond_to?(:key?) && obj.key?(key) || obj.respond_to?(:fetch) && key.is_a?(Integer))
+      obj[key]
+    elsif obj.respond_to?(key)
+      obj.public_send(key)
+    else
+      nil
+    end
+
+    # Convert result to liquid and set context for nested Drops
+    result = result.to_liquid if result.respond_to?(:to_liquid)
+    result.context = context if context && result.respond_to?(:context=)
+    result
+  end
+
+  # === HTML/URL Encoding ===
+  # Capture method references before sandbox locks - only the specific methods we need
+  CGI_ESCAPE_HTML = CGI.method(:escapeHTML)
+  CGI_ESCAPE = CGI.method(:escape)
+  CGI_UNESCAPE = CGI.method(:unescape)
+
+  HTML_ESCAPE_MAP = { '&' => '&amp;', '<' => '&lt;', '>' => '&gt;', '"' => '&quot;', "'" => '&#39;' }.freeze
+
+  def self.escape_html(obj)
+    CGI_ESCAPE_HTML.call(to_s(obj))
+  end
+
+  def self.url_encode(obj)
+    CGI_ESCAPE.call(to_s(obj))
+  end
+
+  def self.url_decode(obj)
+    CGI_UNESCAPE.call(to_s(obj))
+  end
+
+  def self.escape_once(obj)
+    # Only escape if not already escaped
+    to_s(obj).gsub(/["><']|&(?!([a-zA-Z]+|(#\d+));)/, HTML_ESCAPE_MAP)
+  end
+
+  # === Base64 ===
+  # Capture method references before sandbox locks
+  BASE64_ENCODE = Base64.method(:strict_encode64)
+  BASE64_DECODE = Base64.method(:strict_decode64)
+  BASE64_URL_ENCODE = Base64.method(:urlsafe_encode64)
+  BASE64_URL_DECODE = Base64.method(:urlsafe_decode64)
+
+  def self.base64_encode(obj)
+    BASE64_ENCODE.call(to_s(obj))
+  end
+
+  def self.base64_decode(obj)
+    BASE64_DECODE.call(to_s(obj))
+  end
+
+  def self.base64_url_safe_encode(obj)
+    BASE64_URL_ENCODE.call(to_s(obj))
+  end
+
+  def self.base64_url_safe_decode(obj)
+    BASE64_URL_DECODE.call(to_s(obj))
+  end
+
+  # === String Manipulation ===
+
+  def self.strip_html(obj)
+    to_s(obj).gsub(%r{<script.*?</script>|<!--.*?-->|<style.*?</style>}m, '').gsub(/<.*?>/m, '')
+  end
+
+  # Truncate string to length with ellipsis
+  def self.truncate(input, length = 50, ellipsis = '...')
+    str = to_s(input)
+    ell_str = to_s(ellipsis)
+    len = length.to_i
+    l = [len - ell_str.length, 0].max
+    str.length > len ? str[0, l] + ell_str : str
+  end
+
+  # Truncate to word count
+  def self.truncatewords(input, num_words = 15, ellipsis = '...')
+    max_words = [num_words.to_i, 1].max
+    words = to_s(input).split(' ', max_words + 1)
+    if words.length > max_words
+      words[0, max_words].join(' ') + to_s(ellipsis)
+    else
+      input.to_s
+    end
+  end
+
+  # === Date Formatting ===
+
+  def self.date(input, format)
+    return input if format.to_s.empty?
+    d = case input
+    when Time, Date then input
+    when 'now', 'today' then Time.now
+    when /\A\d+\z/, Integer then Time.at(input.to_i)
+    when String then (Time.parse(input) rescue input)
+    else input
+    end
+    d.respond_to?(:strftime) ? d.strftime(format.to_s) : input
+  end
+
+  # === Collection Helpers ===
+
+  # Iterate safely, handling ranges and non-iterables
+  def self.iterate(collection)
+    case collection
+    when Range then collection.to_a
+    when nil then []
+    else collection.respond_to?(:each) ? collection : []
+    end
+  end
+
+  # Get collection length safely
+  def self.size(collection)
+    collection.respond_to?(:size) ? collection.size : 0
+  end
+
+  # === Default Filter ===
+
+  def self.default(input, default_value, allow_false: false)
+    if allow_false
+      (input.nil? || (input.respond_to?(:empty?) && input.empty?)) ? default_value : input
+    else
+      (!truthy?(input) || (input.respond_to?(:empty?) && input.empty?)) ? default_value : input
+    end
+  end
+
+  # === Array/String Slice ===
+
+  def self.slice(input, offset, length = 1)
+    off = to_integer(offset)
+    len = to_integer(length)
+    if input.is_a?(Array)
+      input.slice(off, len) || []
+    else
+      to_s(input).slice(off, len) || ''
+    end
+  end
+end
+
+# Alias for backwards compatibility
+LiquidRuntime = LR