feature/xml2 port

[LiNk-NY]

• migrate XML to xml2
• add unit tests for getHtmlTitle, getDocumentTitles, and getVignetteIndexEntry

[Copilot]

Pull request overview

Ports HTML/XML handling from XML to xml2 by introducing compatibility wrappers and updating title extraction, along with adding RUnit tests and small fixture files to validate behavior.

Changes:

• Added R/xml2_helpers.R implementing xmlNode(), xmlOutputDOM() / xmlTree(), and saveXML() equivalents on top of xml2.
• Updated getHtmlTitle() to parse HTML titles with xml2 instead of XML/XPath helpers.
• Added RUnit tests plus minimal inst/extdata fixtures for vignette title extraction paths.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
R/xml2_helpers.R	Adds xml2-based compatibility wrappers for legacy XML DOM-building and serialization APIs.
R/repository.R	Switches HTML title parsing from XML to xml2.
NAMESPACE	Removes XML imports and adds xml2 imports.
inst/unitTests/test_repository.R	Adds unit tests covering getHtmlTitle, getDocumentTitles, and getVignetteIndexEntry.
inst/extdata/vignette.Rmd	Adds a source vignette fixture containing \\VignetteIndexEntry.
inst/extdata/vignette.html	Adds an HTML fixture with a <title> element.
DESCRIPTION	Replaces XML with xml2 in Imports and updates Collate to include the new helper file.

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