feat(Class::C3::XS): pure-Perl shim so jcpan -t Class::C3::XS passes

[fglock]

Summary

jcpan -t Class::C3::XS was failing 12/13 test files with:

Can't load loadable object for module Class::C3::XS: no Java XS implementation available

Class::C3::XS is XS-only — the dist's lib/Class/C3/XS.pm is just an XSLoader::load(...) plus a few next::* dispatch subs. The four functions it exports map cleanly onto core mro (5.9.5+), so this PR adds a pure-Perl shim and a generalised XSLoader hook to load it.

Changes

New src/main/perl/lib/Class/C3/XS.pm — pure-Perl shim:

• calculateMRO → mro::get_linear_isa($class, 'c3')
• _plsubgen → monotonic counter (only consulted for cache invalidation)
• _calculate_method_dispatch_table → no-op (core mro handles C3 dispatch)
• _nextcan → walks caller() past next:: / maybe::next:: frames, then searches the C3 MRO from the calling package onward
• Also defines next::can, next::method, maybe::next::method (PerlOnJava's MakeMaker skips installing the dist's pm because it's superseded by the bundled jar shim, so these have to live here for goto &next::can in t/36_next_goto.t)

XSLoader.java — when the existing @ISA / ::PP fallback paths don't apply, try loading a bundled jar:PERL5LIB shim of the same name. loadJarShimOverrides now returns boolean to signal whether a shim was actually found. This generalises the existing hook so any future XS-only module can be supported by dropping a .pm into src/main/perl/lib/.

Test plan

• make (full unit suite) — green
• ./jcpan -t Class::C3::XS — Files=13, Tests=46, Result: PASS
• All 13 individual test files in the dist pass: t/01_MRO.t … t/05_MRO.t, t/30_next_method.t … t/36_next_goto.t

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