Plugins persisted domain data via the raw acadrust::xdata API with
boilerplate and no APPID registration. Add read_record / write_record /
remove_record keyed by entity handle; write_record replaces an existing
record for the same app and registers the application in the APPID table
so the data round-trips through DWG/DXF for other CAD apps.
Part of the #100 extensibility epic (surfaced in #106).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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