Component fragments arrive without xmlns:xlmk, emiting libxml warnings into page output

[Nealium104]

Describe the bug
Component fragment files ({id}_ref*.xml) use xlink: prefixed attributes on <extref>, but do not declare the xlink namespace. SimpleXMLElement therefore emits namespace warnings on every parse, and with display_errors enabled those warnings are written to the page. This is a pre-existing bug, before limestone rewrite.

The links still render correctly, and production silences warnings, so this is more of a hazard that should be fixed rather than a bug blocking new versions.

To Reproduce

1. Start a findingaid application
2. Ensure you have the latest XML sample by running make sample.
3. Visit findingaid "Latino Student Union (University of Kentucky) records". ID: xt73r20rv526
4. View exact errors at the top of the page or alternatively, look at logs for the container (possibly via make logs).

These errors are also viewable in production, via docker logs.

Additional context
Line warning: app/Models/Component.php:24