Add caching for Kroki diagrams

[theletterf]

This continues #1599

Overview

This PR implements a diagram caching system to improve performance and reliability for documentation builds. The system provides local SVG caching with fallback to Kroki URLs. Hopefully I steered Claude correctly on this one...

Features

• Automatically downloads and caches diagram SVGs locally during build
• Content-based hashing (SHA256) prevents duplicate downloads
• Meaningful filenames: {markdown-file}-diagram-{type}-{hash}.svg
• Asynchronous, non-blocking downloads using fire-and-forget pattern
• Registry-based tracking of active diagrams during builds
• Automatic cleanup of unused/stale cached files after build completion
• Prevents cache directory bloat over time
• Shared HttpClient singleton prevents socket exhaustion
• Connection reuse across all diagram downloads
• 30-second timeout configuration for reliable network handling
• Atomic file writes prevent corruption during concurrent operations

[github-actions]

🔍 Preview links for changed docs

• docs/syntax/diagrams.md

[theletterf]

@Mpdreamz @reakaleek An experiment. For reasons I don't fully understand, the local cache isn't used by the preview in this PR, but it does work locally.

[theletterf]

@Mpdreamz Thanks! Most changes applied I'd say. Still wonder why the preview doesn't use the cached SVGs though. I guess the proper docs build will?

[Mpdreamz]

Added a few more review comments

[Mpdreamz]

Still wonder why the preview doesn't use the cached SVGs though. I guess the proper docs build will?

Will dig into it at some point this week!