Fix CPAN XML and AnyEvent compiler/tooling support

[fglock]

Summary

• teach the bundled-provider manifest and CPAN resolver about transitive runtime requirements
• add a JAXP-backed XML::LibXSLT provider and declare the XML provider dependency chain
• fix managed nonblocking socket fcntl, connect completion, stream initialization, and accept behavior
• preserve virtual descriptors for anonymous handles created into hash/array lvalues
• retain weakly registered watcher graphs when a strong callback cycle owns the observed watcher

This unblocks Catmandu::CrossRef and AnyEvent::SMTP without new distribution preferences.

System Perl baseline

• Catmandu::CrossRef: passes
• AnyEvent::SMTP: passes
• AI::Pathfinding::AStarNode: fails on system Perl 5.42 (Not a HASH reference), ignored
• Alien::ZMQ: fails on system Perl because libzmq headers/pkg-config and its fallback download are unavailable, ignored
• Games::Quake::Stats: fails on system Perl because GD/GD::Text/GDGraph are unavailable, ignored

Passing dependencies checked separately include Heap::Fibonacci (2473 tests) and String::ShellQuote (33 tests).

Verification

• make — pass, 409 unit files (2 skipped)
• ./jcpan -t Catmandu::CrossRef — pass, 2 files / 9 tests
• ./jcpan -t AnyEvent::SMTP — pass, 11 files / 4 runnable tests; fork and author tests skipped as expected
• system Perl validation for all new Perl behavior tests — pass
• ./jperl src/test/resources/module/XML-LibXSLT/t/basic.t — pass, 3 tests

make test-bundled-modules includes the new XML::LibXSLT test and that test passes. The aggregate local run is currently blocked by pre-existing test-runner cwd failures: 145 tests that use require './t/...', FindBin, or sibling fixture paths cannot locate those resources. The failures are path-resolution errors rather than assertion regressions.