Implement XML::LibXML::Dtd->parse_string for HTML-DTD compatibility

[fglock]

Summary

The HTML-DTD 0.04 test suite fails under PerlOnJava because the bundled
XML::LibXML compatibility layer does not provide
XML::LibXML::Dtd->parse_string.

This is a PerlOnJava API-compatibility gap, not an HTML-DTD implementation
failure.

CPAN evidence

• Distribution: HTML-DTD 0.04
• CPAN run: 20260910-101908-8428
• Failing test: t/validate.t
• System Perl: the complete suite passed after installing HTML-DTD into an
  isolated prefix; the validation test was skipped because the system lacked
  XML::LibXML. An isolated XML::LibXML installation attempt was blocked by a
  missing Alien::Libxml2 prerequisite and unavailable CPAN network access.
• PerlOnJava: five test programs pass; t/validate.t aborts when it calls the
  missing method.

Failure

PerlOnJava reports:

Can't locate object method "parse_string" via package "XML::LibXML::Dtd"

The minimal reproducer is:

use XML::LibXML;

my $dtd = XML::LibXML::Dtd->parse_string('<!ELEMENT root EMPTY>');

This fails under jperl with exit status 255. The same API is used by
HTML-DTD's t/validate.t after obtaining the XHTML DTD text.

Root cause

The bundled Perl implementation declares XML::LibXML::Dtd as a subclass of
XML::LibXML::Node, and the Java implementation wraps document-type nodes,
but no parse_string method is implemented or registered for the Dtd
package.

Expected behavior

XML::LibXML::Dtd->parse_string should parse a DTD string and return a usable
XML::LibXML::Dtd object with behavior compatible with upstream XML::LibXML.

Acceptance criteria

• Add a focused project-owned regression test for
  XML::LibXML::Dtd->parse_string.
• Confirm the expected behavior with system Perl using a usable XML::LibXML
  installation.
• Implement and register the Dtd parsing API in the PerlOnJava XML::LibXML
  compatibility layer.
• Run the focused regression on both JVM and interpreter backends.
• Verify that HTML-DTD 0.04 t/validate.t passes under PerlOnJava.
• Keep existing XML::LibXML compatibility tests green.