Engine: `<%# herb:strict false %>` directive

[marcoroth]

Summary

Add support for a file-level directive to control strict mode in Herb::Engine.

Background

The engine already supports a strict option that gets passed to the parser:

# lib/herb/engine.rb
@strict = properties.fetch(:strict, false)
parse_result = ::Herb.parse(input, track_whitespace: true, strict: @strict)

What strict mode controls

In the parser (src/parser.c), strict mode affects HTML tag matching behavior:

• strict: true - Reports errors for omitted closing tags (e.g., <p>text without </p>)
• strict: false - Silently handles omitted closing tags without errors (following HTML5 optional closing tag rules)

This is useful because HTML5 allows optional closing tags for elements like <p>, <li>, <td>, etc.

Proposed Directive Format

<%# herb:strict false %>  <!-- disable strict mode for this file -->
<%# herb:strict true %>   <!-- enable strict mode for this file -->

Use Cases

1. Legacy templates - Files with valid HTML5 that omit optional closing tags
2. Migration - Gradually enable strict mode across a codebase
3. Third-party templates - Templates that can't be easily modified but are valid HTML

Relationship to Security Directive

This is separate from <%# herb:security false %> (#1161):

Directive	Controls
herb:strict	Parser strictness (omitted closing tag errors)
herb:security	Security validation (injection pattern errors)