Add yearly and monthly sitemap organization for large sites

[Copilot]

This PR implements a new sitemap feature that organizes content by publication year and month, similar to the approach used by Metro.co.uk. This is particularly beneficial for large sites like news websites that host substantial amounts of historical content.

Key Features

Yearly/Monthly Sitemap Organization:

• When enabled, the main sitemap.xml becomes an index that links to yearly or monthly sitemaps
• URLs like sitemap.xml?yyyy=2024 show all posts from 2024
• URLs like sitemap.xml?yyyy=2024&m=12 show posts from December 2024
• Maintains existing query limits and honors all current settings

Benefits:

• Optimized crawl budget - Search engines can focus on newer content while still indexing older content efficiently
• Faster generation - Individual yearly/monthly sitemaps are smaller and generate faster
• Better cache efficiency - Each time period can be cached independently

Implementation Details

Settings & UI:

• Added "Use yearly sitemaps" option (hidden when optimized sitemap is disabled)
• Added "Use monthly sitemaps" sub-option (hidden until yearly is enabled)
• Proper JavaScript visibility toggles following existing patterns
• Both options default to disabled for backward compatibility

Backend Changes:

• Enhanced Registry::_init() to capture and sanitize $_GET parameters
• Modified Base::generate_sitemap() to accept year/month filtering parameters
• Added intelligent date filtering using WordPress date_query for efficient database queries
• Implemented per-period caching with unique cache keys (e.g., base_2024_12)

URL Structure:

# Main sitemap (when yearly enabled) - shows sitemap index
/sitemap.xml

# Yearly sitemap - shows all posts from 2024
/sitemap.xml?yyyy=2024

# Monthly sitemap - shows posts from December 2024
/sitemap.xml?yyyy=2024&m=12

Parameter Sanitization:

• Years: 4-digit format (1900-2099 range)
• Months: 1-2 digit format (1-12 range)
• Invalid parameters are safely ignored

Developer Features

• New public method: \The_SEO_Framework\Sitemap\Registry::get_sitemap_parameters() - Gets sanitized year and month parameters
• New filter: the_seo_framework_sitemap_index_schemas - Filter sitemap index XML schemas
• Backward compatible - No changes to existing functionality when feature is disabled

Testing

All modified files pass PHP syntax validation. The implementation includes comprehensive parameter sanitization with proper regex patterns and follows WordPress coding standards.

Fixes #649.

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