Add WooCommerce integration and extensibility filters

[ilicfilip]

Summary

• Adds a WooCommerce integration that exposes products at .md URLs with price, SKU, stock status, attributes, and variations — fixes WooCommerce Product Pages #32.
• Introduces markdown_alternate_frontmatter and markdown_alternate_content_sections filters so any plugin can extend the markdown output without subclassing. The WooCommerce integration is the reference implementation.
• Adds markdown_alternate_cache_version filter and mixes it into the transient cache key, so integrations pulling from postmeta can invalidate cleanly. Woo also invalidates on woocommerce_update_product and stock-change hooks.

Design notes

• ContentRenderer::generate_frontmatter() now builds an associative data array and passes it through a YAML serializer, so integrations add keys instead of string-munging YAML.
• Body sections are a priority-ordered list (title=0, post_content=100). Integrations can interleave — e.g. Woo places a summary at 10, short description at 50, attributes at 150, variations at 160.
• Woo integration is guarded by class_exists('WooCommerce') in Plugin::register_integrations().

Test plan

• With WooCommerce active, /product/sample.md returns a product page with price/SKU/stock in frontmatter and a summary block in the body
• Variable product renders a ## Variations section listing each variation with attributes + price
• Simple product with no attributes renders without an empty ## Attributes heading
• Updating a product's price invalidates the cached .md output
• Without WooCommerce active, posts and pages still render unchanged
• A third-party filter adding a cook_time frontmatter key and an "Ingredients" section renders correctly