perf: Add inventory caching for interlink resolution

[CybotTM]

Summary

Introduces CachingJsonLoader that decorates JsonLoader to cache inventory files locally, significantly reducing render time for documentation with many interlinks.

Features

• File-based caching: Stores inventory JSON files in system temp directory with configurable TTL (default 1 hour)
• In-memory caching: Deduplicates requests within the same render process
• Parallel prefetch: Capability for bulk inventory loading (enables future parallel fetching)
• Hash-based filenames: Uses xxh128 for URL → filename mapping

Performance Impact

~53% render time improvement for interlink-heavy documentation (measured on TYPO3 core docs).

Technical Details

• Decorator pattern: CachingJsonLoader wraps existing JsonLoader
• PHP 8.1 compatible (no typed constants or #[\Override])
• Cache location: {sys_temp_dir}/typo3-guides-inventory-cache/
• TTL validation prevents stale cache usage

Part of Performance Initiative

This is Phase 1.1 of the performance optimization initiative tracked in #1143.

Upstream dependencies (phpdocumentor/guides):

• #1287 - DocumentEntryNode file path method
• #1288 - DocumentEntryNode isRoot flag
• #1289 - ProjectNode find method
• #1293 - O(1) document lookup optimization

Test plan

• Run render on documentation with many interlinks
• Verify cache files created in temp directory
• Confirm cache hit on subsequent renders
• Test cache expiration after TTL

[CybotTM]

Closing - moved upstream to phpDocumentor/guides PR #1295. The caching logic is not TYPO3-specific and benefits all users of phpDocumentor/guides.