Phase 11: DateTime dependency fixes and map/grep @_ access

[fglock]

Summary

This PR continues Phase 11 of CPAN client support, focusing on DateTime module dependencies.

Fixes

• map/grep blocks accessing outer @_: Code like map { $_[0] } inside a subroutine now correctly accesses the outer @_ instead of an empty array
• B module introspection: B::CV::GV now uses Sub::Util::subname to report actual subroutine names, enabling proper namespace::autoclean behavior
• Scalar::Util::blessed: Now returns undef for unblessed references (was incorrectly returning class name)
• GLOBREFERENCE scalar dereference: $$globref now returns the glob itself (from previous commits)

DateTime Status

DateTime now works in pure Perl mode with manual patches:

PERL_DATETIME_PP=1 ./jperl -MDateTime -e '
  my $dt = DateTime->new(year => 2024, month => 3, day => 15);
  print $dt->ymd;  # 2024-03-15
'

Design Doc Updates

• Added next steps for DateTime: investigate namespace::autoclean behavior difference vs system Perl, enable XS fallback to DateTime.java
• Trimmed old progress reports (884→172 lines) while preserving current work

Test plan

• make passes all unit tests
• map/grep with @_ access works correctly
• DateTime loads in pure Perl mode

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