feat: supplement JSON docs with XML documentation from Godot source

[deevus]

Summary

• Automatically fetches Godot XML documentation from the source tree (via GitHub tarball), streaming HTTP → gzip → tar extraction without writing the full archive to disk
• Parses XML docs with zig-xml to extract tutorials, descriptions, methods, properties, signals, and constants
• Merges XML data into the JSON-based DocDatabase: fills missing descriptions, adds tutorial links, and creates entries for XML-only classes
• Adds GDOC_NO_XML env var to disable XML supplementation (set automatically during zig build test)

How it works

1. On first run (cache empty), after generating the JSON dump, runs godot --version to get the version/commit hash
2. Downloads the source tarball from GitHub (refs/tags/{version}-stable.tar.gz, with commit hash fallback)
3. Streams through gzip decompression and tar extraction, filtering for */doc/classes/*.xml and */modules/*/doc_classes/*.xml
4. Writes ~800 XML files (~5 MB) to ~/.cache/gdoc/xml_docs/ with a .complete version marker
5. Parses each XML file and merges supplemental data into the DocDatabase before generating the markdown cache
6. $DOCS_URL in tutorial links is expanded to https://docs.godotengine.org/en/stable

Godot must be installed. If godot --version fails or the download fails, XML supplementation is silently skipped and gdoc works with JSON-only data. --clear-cache clears everything including XML docs.

New files

• src/source_fetch.zig — Version parsing, tarball URL building, HTTP download, streaming tar extraction, cache markers
• src/XmlDocParser.zig — Godot XML doc parser using zig-xml pull parser

Test plan

• zig build test passes (~0.6s, no network I/O)
• gdoc --clear-cache && gdoc Node2D shows tutorials section from XML docs
• gdoc Node2D.position shows property documentation
• GDOC_NO_XML=1 gdoc --clear-cache && gdoc Node2D works without tutorials (JSON-only)