Add Package::Stash::Conflicts stub and update cpan_client.md

Package::Stash Makefile.PL requires Package::Stash::Conflicts to be
loadable during installation. Add stub with no-op check_conflicts().

Update cpan_client.md with accurate blocking issues based on testing:
- Symbol table dereference and B::Hooks::EndOfScope already FIXED on master
- Package::Stash::Conflicts and parse_args are the real blockers
- File::ShareDir::Install needed for DateTime::Locale

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

Author: fglock

dev/design/cpan_client.md

@@ -528,7 +528,6 @@ cpan> install Module::Name
 - [ ] Phase 10: Further compatibility improvements (low priority)
 - [ ] Phase 11: DateTime dependency chain fixes (blocking)
 
----
 ## Phase 11: DateTime Dependency Investigation (2026-03-19, UPDATED)
 
 ### Problem Statement

src/main/perl/lib/Package/Stash/Conflicts.pm

@@ -0,0 +1,35 @@
+package Package::Stash::Conflicts;
+use strict;
+use warnings;
+
+our $VERSION = '0.40';
+
+# PerlOnJava stub - conflict checking not needed
+# 
+# The original module uses Dist::CheckConflicts to verify there are no
+# conflicting versions of Package::Stash with old versions of:
+# - Class::MOP 1.08
+# - MooseX::Method::Signatures 0.36
+# - MooseX::Role::WithOverloading 0.08
+# - namespace::clean 0.18
+#
+# In PerlOnJava's environment, these old conflicting versions don't exist,
+# so we provide a no-op stub.
+
+sub check_conflicts { }
+
+1;
+
+__END__
+
+=head1 NAME
+
+Package::Stash::Conflicts - PerlOnJava stub for conflict checking
+
+=head1 DESCRIPTION
+
+This is a stub implementation for PerlOnJava. The original module checks
+for conflicting versions of Package::Stash, but this is not needed in
+PerlOnJava's environment.
+
+=cut