docs(xml-libxml): plan for Java-backed XML::LibXML port

[fglock]

Summary

Adds a planning document at dev/modules/xml_libxml.md scoping a
Java-backed re-implementation of XML::LibXML for PerlOnJava.

Plan only — no code changes.

Motivation

jcpan -t XML::Diff currently fails because XML::Diff requires
XML::LibXML 1.56, and XML::LibXML cannot be installed under jperl:

• Its Makefile.PL requires Alien::Libxml2, which expects either a
  system libxml2 via pkg-config or an Alien::Build-installed share
  dir — neither is available.
• Even if Alien were satisfied, LibXML.xs cannot be loaded by the JVM.

Result: Can't locate Alien/Libxml2/ConfigData.pm, dependent test
suites abort on use XML::LibXML;.

Approach

Mirror the XML::Parser port (XMLParserExpat.java, ~2,200 LOC of
JDK SAX glue) with XMLLibXML.java backed by javax.xml.parsers +
org.w3c.dom + javax.xml.xpath. No new Maven/Gradle dependencies
for Tier A.

Tiers (sized in the doc)

Tier	Scope	LOC estimate	Effort
A	XML::Diff unblock (~30 methods)	1,000–1,700	2–4 days
B	"Useful" coverage (~80% of CPAN consumers)	3,000–5,000	1.5–3 weeks
C	Comprehensive (validation, XInclude, SAX, …)	7,000–11,000	1.5–3 months

Upstream XML::LibXML 2.0210 is ~10k .pm + ~5k .xs for reference.

Out of scope for this PR

Implementation. A follow-up branch will scaffold Tier A.

Test plan

• Doc-only change; no build/test impact.
• (Follow-up Tier A PR) jcpan -t XML::Diff passes 38/38; make and make test-bundled-modules stay green.

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